Quick observation regarding carsets vs tracks: heavy carsets seem account for most/all performance dropoff. Simpler carsets barely dropoff during formation laps and first corner or two before the pack spreads out. OTOH the whole library of nice Pavel cars 1990-2001 (lola1990-95, reyn1996-2000) suffer a consistent 10-20 FPS dropoff more or less regardless of track: ie mid 40's/ low 50's, with a rare occasional dip into the mid/high30's. But they do snap back to 60 pretty soon after it goes green and the cars spread out. Maybe the Reyn99 car suffers a bit more than the others? Tracks *might* contribute about 5 FPS? but then again only with the heavy carsets.
IRL carsets (irl96a-b thru irl99a-b etc.: Pavel/Claudio I beleive): negligible fall off - may be based on the simpler 2k model?
Champcar sets (Blackcat indy05 & indy06, CCWS2k6): much larger ~8mb dats and wonky issues. Even reduced won't always load (or load consistently) some reduced tracks (ims2000, sears07, sears08, denver91). And sometimes a save error loading/switching between these cars - and the Houston CTD mentioned before. Also a bigger dropoff to low 30's. But again these aren't true CART era cars.
So right now it looks like carsets have the biggest performance impact pound for pound.
EDIT: Probably the worst combo I've seen is reyn1999 + Clevland (7-23-2012 dat); formation lap never above 30s. Add more info