My specs are below, and I use Starstorm Gfx drivers, but I don't think that has made much difference to GP4 anyway.
I run @ 33FPS in the game setting ( I don't use Fraps in GP4) @ 1280*1024. I have all settings on max except:
Heat haze and video screens off;
Bump mapping off (although turning it on for me lowers PO!);
Mirrors to cars only. I have tweaked the gfx file so that I have the cars, the tracks and grass and barriers only (no smoke or anything either), and the cull distance for the detail and cars is 50m;
Satic environment maps
Reflections are high, but the track reflections in the wet are set to low. This gives you the really reflective track but a much lower PO than leaving it on high where the reflections are less.
I use mainly vipers/enzoj's cars
I don't have a wheel with an LCD screen, and this saves 10% PO.
My PO in the dry @ the start of the race in Europe (which is the most demanding track at the start) is 90% at the back of the grid (only 20cars), and may peak to about 100% at the first corner sometimes.
If it's raining, the PO goes up 20% everywhere.
If it's dry but is going to rain later, the PO goes up 30%. So the only times it goes above 100% is at the start in most tracks in the rain.
I don't have AF, AA or V-sync on. Interestingly though, if I set AA to 4x, I get no PO hit in car, but when I go into trackside camera, the PO will be up to 50% higher than when AA is off. If I turn AF on ,I automatically get a 30% PO hit.
I'll send you my gfx config if you want.