GT-One, thats a good one to try next.
I used your GT40 Test Car setup with the White GT40 and I'm not sure it works as well for that car. I only did an hour test (it was Midnight and I needed to get to bed) and when I test I like to at least give it 2-3 hours to see how it does.
Anyway, I pushed the 1st Bob on the start, got the lead by the 2nd lap (no R8 but there was a GT-One and it seemed almost as fast as the R8). I was mostly checking lap times, tire wear (RS), and gas. First pit was 3 laps, fastest lap time was about 8:12. Driver change on 1st pit, only pushed a little out of the pits, 4 laps and pit, lap times around 8:30ish. I think HP on the White GT is 817 vs the test car's 826, but it shouldnt make that much of a difference. There might be a difference in the weight though, I didnt check that last night.
It did out run the AI driven White GT though, makes me curious what the AI cars setups are, you almost feel sorry for the C-5 Vette and the Lister cars that get lapped about 20 times on the 24hr race.
On the 4hr Nurburgring I like using the Audi R8 LMS car