I just found this link on the High Gear forums:
http://www.racesimcentral.net/
it's a link to a utility called GP3 Advanced Physics Editor. I haven't
tested it yet, but it seems to allow a lot of physics variables to be
changed. Amongst them, it's apparently also possible to change the
optimal slip angles, the only thing I wanted to see tweaked in GP3
actually (the tires appear to be too stiff currently). I can't wait to
get home and try it out!
-Gregor