Ive only played prologue and GT3 and the change from GT3 to GT4P was accompanied by a DFP, ive been using DS2's up until i got Prologue, and now, having gone back to GT3 with my wheel after completing GT4P (All golds including coffee breax, yes even that one with the S2000) I can say the difference in rallying is vast.
The arcade feel of GT3 rallying is fun.
You swoop one way, get the throttle off to aid the turn, then re-apply to turn the other way and swoop that way. Easy. Get it wrong? turn the wheel and floor it. Problem solved.
GT4P is entirely different, the same level of drift can be attained, but the amount of steering input is huge.
The wheel is turning left and right all the time to keep the car straight, and i dont mean small amounts i mean a lot.
When you turn in for a sweeping bend, you need to oversteer, then turn back (opposite lock/countersteer) then turn in again, then repeat, to upset the balance of the car and shift the weight.
This takes some time and lots of practice to even begin to get right, and if you arent using a rally car designed for the dirt then its even harder.
Balancing the car during a drift is made more difficult because of the 900 degrees of steering input you can make, moving the wheel a small amount oftenm results in no direction change at all as you slide, meaning you must put in more turn and then counter steer to get it just right.
I would say its more realistic by far.
But in the real world, it is just added to GT to keep it in line with the other games and their ranges of options.
I would still class it as a part of the whole car, but really its only the headrests, if you see what i mean.
Some would say its essential, and wouldnt buy a car without headrests, some would be too short to care, and buy it without considering them.
I still maintain it is a very rewarding aspect of the game as a whole though, and i will be rallying away as soon as they feel like releasing the game.