We could just say that the RCS bonus is an absolute bonus AFTER all other bonuses are applied. So if your base turn rate is 6, and then you add your Impulse Thrusters skill and your engine power and stuff and come out with a final turn rate of 9, then RCS adds to that, and your 9 becomes a 13. That's a huge bonus for a cruiser. While an Escort, which already at 30 for a final "realized" turn rate, will only get to 34, which is much less amazing.
(We could also just say that "you can only have one of these consoles equipped" for those people concerned about super-agile cruisers that don't mount any armor.)