Although they don't directly enhance firepower the same way that weapons upgrades do, Offensive Refits can greatly increase the ability of your ship to impact a battle. Pairing them up with correct ship and strategy can allow you to make the mos of these upgrades.
Found in: Assault Frigate Mk II
The Good: This gives a bump to your expected damage output vs. squadrons.
The Bad: Only works at close range. So far Offensive Refits are only available on medium and large ships, which may be more efficient when shooting at other ships.
Best on: The Assault Frigate Mk II A can make some use of this upgrade. It combines 2 blue dice with some large firing arcs that can catch a lot of squadrons.
Combos with: Gunnery Teams will allow you to shoot at both squadrons and ships. Ruthless Strategists would allow you to maximize your anti-squadron shooting.
Verdict: It's a good enough card for what it does. Unfortunately, it does a job that really isn't necessary. If your opponent doesn't bring squadrons it is wasted. I'd think this was a more useful card without the range restrictions. I can see where it could be very powerful but it doesn't work in enough instances to make me want to pay for it.
Expanded Hangar Bay
Found in: Core Set, Victory SD
The Good: 5 points is a good buy to add 1 to your ships squadron value. It makes the Squadron command a much more efficient choice.
The Bad: No real drawbacks other than doing nothing when you are using a Squadron command.
Best on: Any ship you are building as a carrier. Almost a mandatory buy in that case.
Combos with: Other upgrades that enhance the Squadron Command like Adar Tallon, Admiral Chiraneau, and Flight Controllers.
Verdict: If you are building a ship as a carrier, than this is a must have upgrade. It allows you to get the most out of your Squadron Commands.
Phylon Q7 Tractor Beams
The standard tractor beam projector found on the Imperial Star Destroyer.
Found in: Imperial SD, MC-80
The Good: This is a great upgrade as it gives you another chance to mess with your opponent each activation. Slowing ships down can help you predict where they will be next and set them up for destruction.
The Bad: It can be countered with Navigate tokens, but even then you are hurting them by forcing the token spend.
Best on: Large ships like the ISD and MC-80 will get the most use form this upgrade as they can effect anyone on the table.
Combos with: Ion Cannon Batteries can strip away the tokens, making the Tractor Beams more effective. Conversely if the Tractor Beams force the Token spend, then the Ion Cannons can do more damage.
Verdict: This is a great upgrade and will have a huge impact on the game. Being able to limit your opponent's movement choices will allow you to setup squadrons to ambush them. You can keep Gladiators from closing as quickly or CR-90s from running away. I am pretty sure I will always have 1 of these in my fleets.