I don't think that is the case. When fine tuning my RB4 gear ratios I noticed that i had a fairly limited rpm range within which the revs would drop for the shift to feel correct, however in the XRT, the torque just pulls from much lower in the rpms. I would love an accurate in-game torque/kilowatt curve to see weather or not I'm imagining things, but I tested using LFS tweak offline, by adding a 6th gear to the RB4 and redoing the gear ratios to suit. End result was not very satisfying, even though the RB4 with 6 gears is capable of keeping up with the FXO and the XRT in terms of top speed and maintaining cornering capability, it just isn't right that the power band for a 4WD turbo feels so small.
P.S. @ Tristan, I'm curious about what you mean when you say "Maybe the cars that have 4WD are also less aerodynamic..."