I would highly suggest you to go with dual (heavy) cannons at the front. In this case turrets are the superior choice at the back, because they will -unlikely any other weapon- increase your forward firepower. Mines are only in special cases really useful, aft beams or torpedos would require you to leave the firing arc of your forward weapons to fire them - a huge loss of firepower and also in most cases not a wise choice.
Therefore: Best choice are usually turrets; nothing fancy, but a decent support for your forward weapons.
Its nice to have a 360 degree firing arc in this ship but the damage is so weak that I feel like all it does is drain power from other weapons.
Should I replace it with mines, a second torpedo, some rear facing cannons...what?
Or is it really helping me?
The way weapons work, and there is some oddities to the system, but in the most basic terms:
First weapon fires at full power.
Second weapon fires at full power and applies its drain
Third weapon fires at the reduced power and drains
And so on
So even if you've got a lot of weapons firing at once and add one more, it's not taking power away from the rest, it's just firing at reduced power. More guns firing is better, it's just that each one is less better than the one before it.
Nagorak has done a lot of testing in weapon firing and damage based on different loads. He has a program which will show your DPS vor various loads and skills. The thread is stickied on this section.
The bottom line is that adding more weapons does more DPS, but it's more linear. You have a diminishing damage curve based on the extra power drain. IIRC the sweet spot seems to be 6 beam weapons. I thin cannons are similar. You will do more damage with 7 or 8 energy weapons, But the DPS curve really flattens out after 6 energy weapons. Any explore Nargoraks thread. I lot of good info if you are a mini-max'er type.
It actually does feel more powerful than I remember so I guess this thread was premature. Before the BoP the last time I used a turret was some ill conceived experimentation with a mark 2 or something.