Have you changed your wheel settings in the setup menu? That doesn't import from GT5.
If it's the oversteer thats killing your lap, I know you've said you have tried active steering but I'd whack that back on and just try to set some consistency laps without pushing, even if it's half throttle and then just focus on each corner, if you crash after that corner keep at that corner and then push through that way.
The only setting that affects the DFGT is power steering (and FFB), isn't it? The other setting is only for the older, 200 degree wheels? Anyway I have that on and I tried various FFB settings, from 2 to 10. I felt the difference, but the outcome was the same.
As for the main settings, all aids off then as I say, AS tried both on and off.
It's weird, I know the car understeers but I just can't turn in where I want with the wheel, I was turning it more than 180 degrees and still not feeling as if I had it pointing where I wanted it, it didn't feel sensitive enough.
Then the oversteer was kicking in, on the second lap I spun at the first corner, ended up pointing back towards the straight. Never did that once with the controller. Also kept doing the same at the right hander after the first chicane.
Maybe I've just got too heavy a foot, but I was trying to stick to 1/2 and 3/4 throttle whenever possible, as you suggest.
I dunno, maybe I just need more time on GT5 with the wheel. As I say, I've played with it on PC sims a lot but it's a pain in the rear setting it up with my PS3 for various reasons, so I don't use it too often. What was weird was it felt great on the other tests, even the other tests with the 370Z.