I agree. However, for the FTO Super Touring Car, it is a completely different and separate car from the FTO LM that appeared in GT3, same goes to the RX-7 LM even though it shares the same name in GT4-GT6 as in GT3 (in GT3 it has a different body, power, weight, and even dimensions from its later GT4-GT6 appearances). Not sure if having the performance of their predecessors could even work here.
On the other hand though, the S2000 LM, Impreza Super Touring Car (formerly known as Impreza LM), and Altezza Touring Car (formerly known as Altezza LM), are the exact same cars as the ones that appeared in GT3, but with their power nerfed (and going from Turbo to NA and 6-speed to 5-speed in the Altezza LM's case) and two of their names changed from LM to Super Touring Car and Touring Car. So I'm all for them having their exact GT3 specs in GT5 Evolution.
If only it was possible to port car models from GT3/GTC/GT4P/GT4 to GT5, we would have the original FTO LM and RX-7 LM (would be '00 or something) being ported into GT5 Evolution alongside their successors as separate cars.