I just hope that GT4 has the sounds of the car modeled accurately. GT3 had a generic sound for every car, very boring. Need for speed hot pursuit has a very very realistic sound engine, everything from the corvette's V8 burble, to the F50's and CGT's F1 wail is covered.
The sound of the SL55 AMG shouldn't be a problem, since Gran Turismo's creator has one.
