getting slightly off topic for a second (partly to stir a few people up), had the weight penalties been more accurately/evenly done then the GTR wouldn't have had the success it did.
back on topic, i think that where fwd gets its advantage in rally is from the fact that, as it was said in the first post (i think), it can pull itself out of the corners, where as rwd has to balance oversteer and understeer. both rwd and fwd will understeer into a corner (pretty easily) when pushed too hard. but if oversteer occurs then the fwd can pull itself out while the rwd has to try and accelerate and control the slide with the wheels that have lost traction. also when running a bit wide (and losing a bit of rear traction) and fwd cars are putting their power down on a 'cleaner' part of the track. all the loose gravel gets pushed to the outside of the track, where the rwd cars are trying to get their drive from.