This is basically what I'm saying. A DFGT is sufficient to max out your potential. I just think you can be more regular on your inputs with a belt-FFB, but that's no big deal.
I think it really depends on your level at the moment you switch from pad to wheel. I got seriously to drive on a 900° wheel on GT4, that makes some years now, but I think I gained around 3 seconds between the moment I left the pad and when I got used to the DFP, though you have to take into account the practice probably also refined my technique.
A better example would be one of the 2014 GT Academy French finalists. He switched from pad to wheel 6 months before, and if I remember well, he had the 2nd French time. He was already insanely fast with a pad before, but he surely maxed out what could be done with it. He just had to get used to the wheel to kick off what he needed to qualify, which I estimate it to be roughly between 1 and 1.5 second.
I think it is easier to practice some things with the wheel, but the most important gain with it is the pleasure increase. A pad is nowhere near a wheel on that point.