Capital ship strength is one of the most controversial debates in SINS. Some want then to be able to smash frigates and others like them fine the way they are. Early game they are fine, but later on in the game the debate over their strength comes up. Giving them a flat bonus in strength would unblance the early game.
What I would like to propose is that the logistic reseaarch for capitals provides more than capital slots. My idea would be for each level researched reduce the fleet cost of capital ships. The goal is that by the time all 16 capital slots are unlocked, capitals cost 0 fleet capacity.
If each stage is kept small, only allowing an extra frigate or so each level then it would not be overpowered early game while making capital ships a better deal for those who fully research them.
If reducing fleet capacity cost is not enough, then build cost could be reduced a bit for each level researched as well.
This idea would encourage the use of capital ships while not changing their current balance in combat.