I think there are two parts in this problem:
1. The mix of engine/exhaust could be improved, as well as boosting certain frequenzies (like bass).
2. The engine sounds decreases in game as other sounds go louder (wind, tire screech, other cars etc).
Now, focusing on the second part, I think the problem is the fact that in real life the dB goes up when a sound gets louder. In GT5 it doesn't, it's mostly other sounds that decrease in volume when a certain sound gets louder. Like this:
1. Here is the recording of an engine. As the RMP increase it gets louder. (Axis 0-35 = loudness, axis 1-10 = time. All values are purely fictional)
2. As the car accelerates, the wind noise will increase as well.
3. Now, you can't have the levels going up like this in a game because you can't go higher than 100% level , or you'll get a distorted overdrive sound. This means that the different sounds would have to share the 100% level between themselves. So as one sound increase in proportion, the other sound would have to go down. In the beginning for instance, there is no wind sound, only engine. So the engine gets almost all of the 100% to play with. As the speed increases, the wind takes more and more room, and although the engine in fact gets louder and louder, its proportion of the total sound space decreases and so does its in game volume. This, I guess, adds to the sensation of driving a vaccuum cleaner.
(Of course, when there's not so much sound, not all 100% of the level range is in use but it's too complicated to make a graph of that...)
4. Now, wind and engine are only two of the sounds that's heard in a race. Here is an addition of a whole bunch of other sounds. The sounds gets louder and louder as speed increases. At time 6 to 10 the sound of another car is heard, probably an overtake.
5. Here is the same situation in game, where the sounds would have to share the same space. Notice how your own engine sound gets smaller proportion as you overtake the other car, and then it climbs up again once you've passed. This is pretty much what happens in game. You can notice this when you're crashing into a wall - instead of producing a loud crash sound it mutes all other sounds.
It's probably a lot more complicated than this, but it's a rough sketch...
Maybe dedicating a certain % of the level range to purely engine sounds could be a way to fix this. That way you would always get a good engine level and the other sounds would only compete with each other - not with the engine.