Designing a fighter with higher speed than max B-FCS tracking speed will result in that excess speed being "wasted" for the purposes of target tracking, yes.
It is not optimal to design to B-FCS tracking speed, in fact the opposite is true, it is optimal to design for racial tracking speed. B-FCS max tracking is 4x the Racial Tracking Tech, however when your ship goes slower than your Racial Tracking and your B-FCS is at least equal to that tracking tech, your weapons will track at the Racial Tracking tech rather than your ship's actual speed. So you can save weight in Engines and Fire Control by forgoing speed... at least for your Beam Fighters.
These will make poor offensive machines, mind you... and designing ships the way you have been would be considered efficient, as it makes the most out of the speed that you have already paid for in engines and fuel. It is still, however, still very useful to build more speed into your designs than you need for a few reasons:
- Future proofing, in C# you can now refit fighters, so swapping out an FCS is trivial.
- Range, with more speed you can dictate the terms of engagement, effectively choosing not only when and where, but also if you want to engage the enemy.
- Evasion, "In combat speed is life, ya go slow... ya die." Especially true for fighters which can't effectively mount heavy defenses.
- Exposure, the less time you spend in range of their guns, the less damage you take. Paired with the improved evasion conferred by it and speed can prove to be vital asset to any fighter craft.