I cant say for sure how much game time I've had with the DFGT but I'm definately still a little slower with the wheel, 'course I've always used controller all the way through from GT1 to GT4 and start of GT5. More consistent with the wheel though. I've had the wheel for about 4 months now but I only play a few(maybe 6 or 7)hrs. a week, on a good week. The thing is, it's hard to do "crazy-go for broke" laps with the wheel 'cause I'm always afraid of spinning out. It's much easier to catch slides with the controller for me. With wheel I always seem to end up spinning or overcorrecting and spinning in the opposite direction. I think It's even made worse for me 'cause of my driving style with the controller. I used to do a lot of trail-braking going into corners and with the fact that this is harder to catch with the wheel I need to totally change my driving style but it's imprinted into my brain and it's hard to get out. Trail-braking is using the brakes to help turn the car. I could brake later, get it turned and then power out straighter. Now, with the wheel, I have to brake sooner, in a straight line, so I have to re-learn all the braking points and then drive the corner smoother with the throttle and steering and my lines are totally different. The style with the controller was more of a fast in, point and shoot affair. Mind you, I knew this was not the fastest way around the track but I was pretty competitive doing it even though most of the faster guys out there with wheels could easily beat me and I always new why they were faster and the wheel just multiply's that fault in driving style.
Best example I can think of is I used to compete in a league with the RUF RGT (Porshe rear engine) which a lot of people hated driving 'cause of the rear weight bias, but it was perfect for my style with the controller and my favorite car for quite some time at around 550pp. Killer under braking and the weight over the rear allowed me to brake late and put the power down early. Now with the wheel, I absolutely hate driving it. I'm all over the place trying to catch the rear end.
Also have too re-tune for the wheel as with my set-ups for the controller I seem to get a lot more understeer. Especially brake bias and diff settings.
Point is, the wheel is a lot more realistic and has pointed out serious faults in my old driving style. It's almost like a whole new game for me. I even went back and did most of the lisence test over and some of the easier A-spec Races.
Sorry for the long post, I tend to get long winded with my explanations.