MHO:

The fact that it is on exit only, I'd suggest looking at the rear toe. You may have too much toe out or too little toe in depending on perspective. Go 1/16 to 3/32" more toe in than whatever you have now and try it out.

The other thing to consider is through longer corners the outside rear coil spring or shock may be bottoming, are you hitting the bumpstops? ANy of those will make your effective rear spring rate go very high very quickly=oversteer.