I was a long time pad user on GT5P, then got a G25, sold the G25 and went back to the pad and now have a Fanatec GT3RS CS wheel & pedals...
Typically a pad is around 1-2 seconds slower on a track like Fuji, I was a pretty quick driver with the pad and in most TT's on here I'd be around the top of the pad users, even beating some wheel users, which was nice
Now I've got a decent wheel though, the pad feels horrible, it has no feeling what-so-ever. Some things are easier on a pad, like if you start to oversteer it's much easier to catch the slide with a pad as lock-to-lock is a slit second compared to several seconds with a 900 degree wheel, but overall being consistent and smooth (which is the key to going quick) is great with a wheel.
Doing the Nordschleife AMG SLS challenges and it makes me very glad I've got a wheel, the FFB you get from the bumps, the kerbs, the front wheels going very slightly airborne and the lack of grip you feel (and the following yank on the wheel from the tyres gripping again!), and of course being able to feel oversteer and understeer, all things you'll never get from a pad
