I'm sure there's a bit of that, but if you try holding the stick full over on GT5/6, it garners noticeable understeer when you do this, and the front will just start to plow. It definitely does not plow in FM5, even if @ 10/10's you might be a tiny bit faster with more precise movements. With a wheel on both games, you can feel when you've turned the wheel to far and lose grip, but its certainly more noticeable in FM5. I also suspect a bit of stability control with the joystick in FM5, as it seems a bit too easy to get a car into a perfect drift, where with the wheel I am having to use a lot more throttle control.
Again, I understand the decision Turn10 made - to make it accessible to driving fans who either don't have the the space, or want to bother with setting up a wheel, and allow them to be competitive, but concessions have certainly been made to the controller user; it's very, very obvious when switching from a wheel to gamepad and vice versa.