Here's my experience at the 'Ring with the less powerful old RUF (don't remember the name, but not the yellowbird) on n2 tyres:
DS2: With the controller it's automatically got the assisted steering on, otherwise we wouldn't be able to keep a car like this on the road. You turn in, if it oversteers, just flick the anologue stick the other way, and it'll be fine. If you get into a slide, the car just seems to float across the tarmac, and accurate, quick correction is not really needed. Bumps don't seem to unsettle the car too badly, although you still have to make sure you're driving as smoothly as possible to keep the car balanced of course.
DFP: Switching to the DFP with steering assistance off, it was completely different. Despite driving fairly smoothly with the DFP, I was fighting with the steering all the time, over bumps, into turns, out of turns. As I turned in, if I didn't get it just right, the back would come out quickly and require a quick and accurate correction. I would occasionally overcorrect, then the car would flick back in the other direction, and I would have to correct again, trying to keep the car under control. However, the extra control of the wheel did allow me to keep roughly the lines I wanted (actually did Miss-Hit-Miss right for example). By the end of the lap though, I was quite drained, and had a big grin on my face. I was expecting the car to react in a similar way to how it did with the DS2, because I thought that was how the game was. But using the DFP, I really felt as if I could have been driving the car around the 'Ring.
For quite a while, my laptimes were actually slower with the DFP than DS2, due to the extra work required keeping the car on the track without steering assistance on. I can now match my DS2 times at the 'Ring in this car with the DFP, but basically you're having to do all the work with the wheel that the playstation is doing for you with the assisted steering when using the DS2. I suppose on smoother tracks, and with more forgiving cars, it would be easier to go more quickly with the DFP compared to the DS2, as you're more concerned with keeping very smooth inputs. But with the RUF at the 'Ring, you're more worried about not getting thrown off the track.
I was always a bit dissapointed with how the car I was driving looked during replays, too. Of course, the replays look stunning, but the car would just seem to float across the surface slightly, not really reacting to bumps, and smoothly moving into and out of oversteer, no matter what the car. Then looking at the replay of the RUF at the 'Ring driven with the DFP, you can see how hard you're having to work to keep it on the track. Just looked a lot better than the lap driven with the DS2.
I hope that helps. I'm sure it's been discussed before somewhere, but I thought I'd add my opinion on this...
