The main problem is most likely not with the physics, it's with the sensitivity of the steering. Even at -2, it's still a bit too sensitive compared to real cars, but at -2, the cars are much easier to drive. I think they just need to add more negative sensitivity values to choose from. This has been an issue since GT4 using the DFP, in GT5 they added the sensitivity adjustment which helped but it still isn't quite enough. I really don't see how hard it could be to match the steering 1:1 on most cars anyways, especially modern road cars if you're using a wheel that turns 900+ degs.
All this said, if you're experienced enough you'll know when you're applying too much steering input based on what the car is doing, so any issues with the steering sensitivity are most likely still driver induced. I say this because i have a decent amount of real-world racing experience and although i haven't driven every car in the OP, the cars i have driven haven't felt wrong to me other than the sensitivity issue, and that issue sticks out and is quite obvious if you've driven a car in real life as well as in GT. I used to drive an S2K as a daily driver in real life and in the game, the steering inputs required are much less than in the actual car, it's the very first thing i noticed since it was the first car i bought after the Fit RS, it's the same with any other car i've driven both in the real world and in the game and probably the same for everything else.
EDIT: This was meant for a different thread, but this actually still applies to the R8.