Actually, the cannons would fit more on Nova, then Vesta. If I good remember the fluff, then modified Nova was a original Defiant proposition. Now it is called "Defiant Pathfinder" or something like this.
No I feel that Nova Fleet ships would be well suited for dual or heavy Cannons. I use cannons on my ship along with a dual beam. The results for me is great. I can do more damage to ships more cause of my cannons to knock out shields. But the size and efficiency of the nova is great and can be a good combat with escorts. I don't see whats the problem with having a smaller ship be able to equip cannons. If a big ship like a Vesta can then why not a Nova? Trust me I think dual or heavy cannons would work for anyone not just science officers.
I think they should do something for the fleet version of the ship, but I'm not 100% sure it should be cannons. Personally, it annoys me that the Vesta has cannons... and I'm a sci officer who uses sci ships.
The other side of me can concede to your point that if they Vesta can, why not the Nova? That the Nova is smaller isn't a knock against that argument, given that most escorts are smaller.
Overall, though, I wish the devs would just figure out cooler ways to make science ships more viable, especially a retrofit Nova, given the fact that Nova class ships are supposed to be highly advanced ships according to cannon. My vote would go for specialized versions of Subsystem Targetting that had different effects than other science ships, like a dot on shields for Sub Target: shields, or a snare for engines, etc. Heck, if they had cool particle effects, I may not even care whether or not they're all that much better than the current borderline useless subsystem targetting powers for science ships.
I mean the Vesta Class is good but I think a science ship would be more suited to have cannons would be the Nova class fleet ship. I am testing it out with normal cannons you can put on any ship, So far the results are great in my opinion. I am able to do more damage to shields and fight against e.g. the borg more easily than with full on phasers. So please if you can think of it may you please make Novas carry heavy cannons and such.
If you want a science ship with cannons, fly the hegtha bird of prey or vesta :p
Pure blood sci ships get no cannons
[Combat (Self)] Your Bite deals 2378 (1475) Physical Damage(Critical) to Spawnmother.
Regarding the Defiant Pathfinder, this "original" Defiant is more a case of the drawing boards for the series production rather than in-universe testing of the Defiant design. As such, it's not something that would be applicable here. The Nova class was a re-use of that design to have a new ship for Voy, in much the same way as some of the ships like the "Merchantman", the Talarian/Acamarian ships, Pakled/Bajoran ships and the freighters, like Bok Nor and others were all re-uses of the same models without necessarily implying any shared design lineage.
It's all about DBBs on the Nova anyhow. With lower ranked tactical spaces, you can apply more powers to them, and you can also make use of the subsystem targeting. Also, a wider arc for the turn rate, which while good, isn't on a par with most escorts.