I'm in the same boat here, exactly the same problems in all the same areas. The slower the speed, the worse the corner exit oversteer is. The car looks like its diffing out, loses all forward drive and spins inner rear.
I will admit I haven't tried 2k rear diff oil on a medium size track yet. Only small tracks. This makes the understeer worse and does nothing for the on power oversteer. Setup helps with drivability a bit, and I can drive around the problem, but it does nothing for improving the laptimes.
Things I have found to help are:
1mm shims under the inner ball studs
1.3mm bars front and rear (need to try 1.4 front and rear)
5.4 front droop
4.4 rear droop
(any less droop ie 6f 5r = diffing out)
Foam bits in the shock caps
This makes the car driveable, not fast. I still get left behind on tight corners.
I will either try a 2mm chassis or get a new car and get my friend to build it. I swear that guy has magic fingers. Everytime he builds a new car they are fast. Everytime I build a new car, slow
