I've done a bit more testing with my Nebula and Kar'fi, trying to find the optimum torpedo setup.

WARNING, LONG MATH-FILLED POST INCOMING: SKIP TO THE END IF YOU HATE MATH
Assuming that as-per Cygone's testing, 3 Photons and 2 high-end DOFFs gives ~148 Torps over 300 secs...

__Base Damage__:

MkXII Photon CrtD/CrtH/Borg: 3048+1000

MkXII Disruptor DHC Acc/CrtH/Borg: 392 (+7.5% chance of 1000)

MkXII Disruptor DBB Acc/CrtH/Borg: 293 (+7.5% chance of 1000)

__Standard DHC firing sequence__ = 2 shots over 1 second, rest for 2 sec.

Base damage is therefore 392*2/3= 261 DPS average.

Extra Borg damage is calculated once per firing cycle = 1000*0.075/3= 25 DPS average.

__Standard DBB firing sequence__ = 4 shots over 4 seconds, rest for 1 sec.

Base damage is therefore 293*4/5= 234 DPS average.

Extra Borg damage is calculated once per firing cycle = 1000*0.075/5= 15 DPS average.

__My Nebula's Power levels__:

Minimum: 52/25 (Normal+Passive bonuses) or 75/25 (with EPTW1)

Standard: 101/90 (Normal) or 125/90 (with EPTW1)

[Every energy point over 50 increases DPS by 2%]
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*** __Average Damage expected from 3x Photons + 2 Purple DOFFs over 300 Secs__:

148 * 4048 = 599104 or ~1997 DPS

*** __Average Damage expected from 1x DHC over 300 Secs__:

DHC at 52 Power (minimum): 300*1.04*261=81432 plus 25*300=7500 = 88932 or 296.44 DPS

DHC at 75 Power (min+EPTW): 300*1.5*261=117450 plus 25*300=7500 = 124950 or 416.5 DPS

DHC at 125 Power (maximum): 300*2.5*261=195750 plus 25*300=7500 = 203250 or 677.5 DPS

>>> How many Photon shots added to the DHC damage would it take to match up with 3x Photons?

At 52 Weapons Power: (X * 4048) + 88932 = 559104 ..... X = 116.15 shots

At 75 Weapons Power: (X * 4048) + 124950 = 559104 ..... X = 107.25 shots

At 125 Weapons Power: (X * 4048) + 203250 = 559104 ..... X = 87.91 shots

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*** __Average Damage expected from 1x DBB over 300 Secs__:

DBB at 52 Power (minimum): 300*1.04*234=73008 plus 15*300=4500 = 77508 or 258.36 DPS

DBB at 75 Power (min+EPTW): 300*1.5*234=105300 plus 15*300=4500 = 109800 or 366 DPS

DBB at 125 Power (maximum): 300*2.5*234=175500 plus 15*300=4500 = 180000 or 600 DPS

>>> How many Photon shots added to the DBB damage would it take to match up with 3x Photons?

At 52 Weapons Power: (X * 4048) + 77508 = 559104 ..... X = 118.97 shots

At 75 Weapons Power: (X * 4048) + 109800 = 559104 ..... X = 110.99 shots

At 125 Weapons Power: (X * 4048) + 180000 = 559104 ..... X = 93.65 shots
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__What about adding turrets in the aft weapon slots?__
Naturally, with more than one energy weapon firing you'll lose some weapon energy, so with 3 aft turrets as well as a forward energy weapon, the actual energy setting your fore weapon fires at will often be a little lower. Every turret over 1 drains weapons energy by 8 per firing sequence, so with 3 turrets firing, and every turret above 1 draining 8 power, the worst case scenario is that your actual weapons power will be sitting at an average of 16 power lower.

However, a turret's standard firing sequence = 2 shots over 1 seconds, rest for 1 sec.

That means that turrets actually fire for 50% of the time. They reserve energy when they're firing, and release it during the recharge cycle, so the "best case scenario" is that 2 turrets will never draw any weapons power (while one rests the other fires) and 3 turrets will only drain 8 weapons power 50% of the time... so you'd be sitting at an average of 4 power lower.

In practice, it's usually going to be either 0 or 8, and very very rarely 16.

Let's assume we're having bad luck, and it's sitting at -8.

***

__This adjusts the DHC values to:__
DHC at 44 Power (minimum): 300*0.88*261=68904 plus 25*300=7500 = 76404 or 254.68 DPS

DHC at 67 Power (minimum): 300*1.34*261=104922 plus 25*300=7500 = 112422 or 374.74 DPS

DHC at 117 Power (minimum): 300*2.34*261=183222 plus 25*300=7500 = 190722 or 635.74 DPS

>>> How many Photon shots added to the DHC damage would it take to match up with 3x Photons?

At 44 Weapons Power: (X * 4048) + 76404 = 559104 ..... X = 119.24 shots

At 67 Weapons Power: (X * 4048) + 112422 = 559104 ..... X = 110.35 shots

At 117 Weapons Power: (X * 4048) + 190722 = 559104 ..... X = 91.00 shots

***

__And adjusts the DBB values to__:

DBB at 44 Power (minimum): 300*0.88*234=61776 plus 15*300=4500 = 66276 or 220.92 DPS

DBB at 67 Power (minimum): 300*1.34*234=94068 plus 15*300=4500 = 98568 or 328.56 DPS

DBB at 117 Power (minimum): 300*2.34*234=164268 plus 15*300=4500 = 167768 or 559.23 DPS

>>> How many Photon shots added to the DBB damage would it take to match up with 3x Photons?

At 44 Weapons Power: (X * 4048) + 66276 = 559104 ..... X = 121.75 shots

At 67 Weapons Power: (X * 4048) + 98568 = 559104 ..... X = 113.77 shots

At 117 Weapons Power: (X * 4048) + 167768 = 559104 ..... X = 96.67 shots

**So with 2x EPTW1 and 3 rear turrets, [2x Photons plus a DBB] beats [3x Photons] under the following conditions**:

__with minimum energy (75/25 with EPTW1)__: when Photon shots fired over 300 secs is __114 or higher__.

__with maximum energy (125/90 with EPTW1)__: when Photon shots fired over 300 secs is **97 or higher**.
__from Cygone's shots-per-300-seconds testing__:

2 Photon Launchers, 1 Purple DOFF, 1 Blue DOFF gives 111 Torps over 300 secs

2 Photon Launchers, 1 Purple DOFF, 2 Blue DOFFs gives 126 Torps over 300 secs

2 Photon Launchers, 2 Purple DOFFs isn't listed, but I'd guess it'd give somewhere around 115-120 torps.

My reasoning for this is that Photon Torpedo firing sequence = one shot over 0.5 seconds, 6 second recharge.

Blue DOFFs give 20% chance for -4 secs recharge and Purple DOFFs give 20% chance for -5 secs.

Note also that all that is before you take BOFF buffs into account. As it stands you'll be getting a short energy damage buff every 30 seconds from EPTW1, and if you're running Tac Team then your Energy weapons will get a slightly better buff than your Projectiles due to the "Starship Energy Weapons Training" and "Starship Projectile Weapons Training" buffs not affecting damage from [Borg] weapon modifiers. And if the Fore DBB you pick is [Disruptor] it'll be contributing more to the damage resistance debuff proc uptime.

And then there are the inherent Beam-Target-Subsystem abilities which are available to you if you have a beam array.

The downside is Tac Consoles. The Nebula only gets two of these, but if you devote them to Torpedo Damage you'll obviously be buffing the damage of your Photon Launchers but not your DBB. This will greatly depend on your chosen captain skills, but I suspect that with 3x Turrets plus a DBB and 2 Launchers the sweet spot would slightly favour 1x Energy Console and 1x Photon Console over 2x Photon Consoles.

In any case, if you're running at 125 Weapons power the DBB adds so much extra damage over the 3rd Photon that you'd be hard pressed to make up the difference using Tactical Consoles alone. For DHCs instead of DBBs, (e.g. ships like the Kar'fi) the difference is even more pronounced, though this would be lessened with a 4th Fore Weapon slot containing another DHC (due to the extra weapons power drain).

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__TL;DR:__
From the math; assuming that you're running at a decent weapons energy setting and using at least two Projectile Weapons DOFFs of Blue Quality or better; it looks like [1xDBB and 2x Photons] are better DPS than [3x Photons] even against bare hull. (And naturally, it's far better DPS against anything with shields...)

__On paper then, I'd assume the best Nebula DPS setup would be__:

+ Weapons Power at 125.

+ [2x Photon Launchers, 1x Disruptor DBB, 3x Disruptor Turrets]

+ [2x Purple Projectile Weapons DOFFs]

+ [1x Purple Gravimetric Scientist DOFF]

+ [2x Blue Photonic Studies Student DOFF]

+ Either [1x Disruptor and 1x Photon] or [2x Photon] Tactical Consoles

+ [4x Particle Generator] Science Consoles

---> Eng BOFFs take EPTW1x2

---> Tac BOFF takes TT1 or HYT1, Attack Pattern Beta 1

---> Sci BOFFs takes GW1, PSW3

(as previously mentioned, I don't personally like the non-ranged aspect of PSW3, so I'll be taking GW3 instead and dropping the two Photonic DOFFs for a Purple Conn Officer, for added Tac Team 1 uptime, and a Purple Shield Distribution Officer, for turning 'Brace for Impact' into 'Reverse Shield Polarity')

Whether or not this translates into much extra real-world DPS is another story, but I think this is how I'll be building my Nebula.