Do you already have a 6 degree rear beam? That will help with the handling too. It will mostly cure understeer, but it also makes the rear less snappy. With less understeer from the suspension geometry you are able to run softer rear springs which might help with the oversteering at high speed.

Do you have some aerodynamic help at the front already, like a Rouse splitter? As I wouldn't expect the aero of the standard car to be such that it gives oversteer at high speed.