My dad's car, a Fiat Marea, has an engine quite similar to the one in the Fiat Coupé. It has a delicious sound in real life, full of character due to the 5cyl. layout, and the game reproduces it very well, astonishingly.
Not even close, I have a 95 Z28 (Magnaflow cat-back) and 95 Trans Am (Hooker cat-back) and my cars sound so much better and ballzy than the 97 Z28 in the game. I find it frustrating PD assigned hoover vacuum cleaner noise to the V8 LT1s and LS1s.
I have a 1998 Jeep Cherokee with aftermarket pacesetter exhaust, and to tell you, it sounds much better then the one in GT5. That's because, well...there isn't one in GT5.