Don't bother with gimballed/turreted cannons, I say as a long time Cannon user.
The point of Cannons is to deal massive burst damage while manuevering. Gimballed/turreted variants sacrifice your damage and velocity. This goes against the purpose of cannons - if you don't want, or cannot use a fixed cannon, then don't bother at all and just get a multicannon.
For special effects:
Dispersal field is good, it's basically chaff. Has no downsides, doesn't even decrease damage or anything.
Autoloader, amazing mod if you're going full-cannon. Since your cannons are getting reloaded while you're shooting them, your clip size is basically doubled and thus your overall DPS is increased aswell.
Force shell. I didn't use this myself - it adds jitter and jitter is rather bad for cannons. It scales with hardpoint size, but there's a maximum limit of force that you can apply, so more than one force shell cannon is useless. Also, it doesn't really do much against targets that are in FA ON mode, but it confuses FA OFF people quite a lot.
Thermal cascade. Good against anything that uses silent running or banks without heat sinks. Also good against people with overcharged powerplants, DD5s and overcharged incendiary weapons.
High-yield, decreases your total DPS by a small bit, but creates a AOE effect. Good for destroying more modules at once, requires less precision for module sniping than a regular Cannon. Good utility, but since shields are resistant to explosive damage, HY is bad if you're flying a full-cannon ship.
