Autoxed my GXP for the first time yesterday and I too ran into the understeer problem. The only thing that seemed to help was to mash the throttle in places where I could which would counteract the understeer with a bit of power oversteer. However my car is bone stock, has not yet had an alignment (there is almost no camber in it now) and was on old Kumhos so there's obviously a lot more I can do. However I am worried after the absolute pasting Kevin Dietz and Fast Mike gave me in their Z0K's. They were both in the 40.9s and my best was a 43.3. The only good thing is that almost no one else could keep up with them either but it's not a good sign if I get straight-timed by that much. Any ideas?