dave: nice idea, but not really there yet. keep it coming
I think a fairly simple and cpu-friendy way to make the engine sound better would be having two layered "channels" instead of one.
The other could be the exhaust simulation as it is currently.
The higher frequencies could be produced with noise generator and / or sample, that could be filtered according to different engines / intake systems.
This way the engine pulse sample itselft wouldn't have to change during game, but we would still have more harmonics.
I believe making engine volume relative to revs would also be very simple. Just use some pre-set or adjustable curve for amplitude vs. rpm. Saturating the engine sound after some point in the rpm-curve could be the answer to more "scream".