Have the turret tracking speed be additive to the ship's tracking speed, not a replacement.
The following scenario has been bugging me for a long time, and the solution just clicked in: Let's say I've got a ship with a speed of 10,000 and my fire-control speed and turret speed are both 4,000. If I want to have a laser mount with a tracking speed of 12,000 (let's say I know that's an enemy's standard speed), then I need to pay for a 30% turret (3x4,000) to mount it in, of which only the last 5% does me any good - the first 25% goes to matching the ship's speed. If the turret speed added to the ship speed, however, then I'd only have to design a 5% turret, since I would only need the 2,000 extra speed between 12,000 and 10,000.
For the case of ship speeds that are slower than the fire-control speed, I would only add turret speed to the ship speed, not to the fire control speed. So a 4,000 turret on a 2,000 ship with 4,000 fire-control speed would result in a tracking speed of 6,000 rather than 8,000. This is because the fire control speed effect is intended as a floor for tracking speed in the case of fixed mounts on slow ships.
I realize you might not want to do this for balance issues, which is fine, but I like the idea of giving an incentive to build fast escorts even if they're equipped with turrets. The thing that triggered this was a thread about using fighters and GB in an anti-missile role - the turret mass penalty disproportionately hits small, fast craft because they have to "waste" a lot of mass getting the turrets' tracking speed up to their own speed. This makes the fire-control bonus that fighters get a lot less effective, since the weapons often won't be able to match the tracking speed of a fighter fire-control.
John