Having this kind of expectations in GT5 is crazy.
Look how screwed up GT5 is - example:
R32 Golf
In real life 'charging" (Super / turbo charging) one of these is very popular, there are many companies offering conversions easily pushing out 500+bhp (or ALOT more).
We even have one in the game (HPA) - but by an MK IV R32 Golf from the used dealer and you can't even put a low or mid turbo kit on it. You get around 400-450 (something) max bhp..??
When there's glaring mistakes and inconsistencies like this in the game, does it really surpirse you that in GT5 not all cars are not going to reach the potential they do in real life??
Try using the 350 Z for track racing, especially the Gran Turismo 350Z RS - caliing this car "slow" is laughable, it's ridiculously quick when dialled in right.
Likewise, try driving a "pro street" car (or car like this) around a track, pffffft....... trans cammer anyone...
There's not point using a spoon to cut bread, or a fork to eat soup.. The 350Z's maybe be dissapointing down the quarter, but try some of these around a track and you'll find it's a completely different story.