Horsepower: about 600
Weight: 1050
Difference between the GT3 and GT4 versions: the GT3 version is more powerful but has a more simple design. Colors are different.
The GT3 version features more powerful engine and sleek body lines. I would rather replace it with the current RX-7 LM of GT4 as that edition is just replica of GT300 race car which is already featured in last few GT games.
Honestly, LM racecars in GT3 were better than those in GT4 for me, especially FTO.
I still have no idea why PD chose to detune them (ex. S2000, Altezza, etc.).
Probably because they tend to be more realistic that way. The GT3 FTO LM is an interesting car but completely ridiculous in terms of common sense. The Altezza LM and S2000 LM are more forgiving but in general they were all overly overpowered and visually pretentious to be part of the game that aims towards realism.
Same goes for the circuits. That is why they were all given flat surfaces and more concrete.