I just got my G25 yesterday and, tbh, the thing that has improved most is my throttle control - I'm gaining half seconds on every corner because I'm not having to pump/tap the button (I could never get the hang of the joystick for throttle)
The wheel is fine and the force feedback is awesome but it hasn't really done as much to help my lap times, since I kinda 'feel' what the car is doing even without the wheel if that makes any kind of sense.
There seems to be a kinda - this guys using a sixaxis - detector in the game that allows you to tap the direction stick at different speeds to account for how much you're turning, kinda like tweening between keyframes if you ever did that sorta thing.
But yeah - the pedals have changed my life!