Not really possible, no. Sure there are obviously a few variables for each engine that make them sound as they do and you could no doubt edit these in memory.
Cranking up the raspiness had a major floor, that being that it still didn't make the engines sound more like, well, engines (while S2 sounds do), and it would easily clip the signal, thus resulting in a horrible sound as soon as you revved it. In fact even now if you turn engine volume to 20 it sounds horrible in the race cars as it's just clipping all over the place.
With all the sound adjustments we had it would be nice to adjust the normal game sounds, such as the menu click and stuff. I mean by the time I've got the sound up loud enough to be like a real engine, that damn click as I go through the menus deafens me!