Meeb,
Think about it ! I think you are missing the point !
15 Hours it may be but if you are doing an FAA CFI course at a school that has JAA approval and the CFI teaching you has FAA/JAA licenses then your 15 hours will probably be part of your FI training
Moral of the story if you want to teach both FAA and JAA then gain your flight instructors rating at a school that teaches both, and it is easier to gain an FAA instructors rating and add JAA then to try the other way around (basically because the FAA are a hell of a lot more logical than good old CAA - Bless their little hearts !)