I wouldnt go more than 7k in the center. Try running heavier sway bars so you dont roll so much weight onto the outside front. The inside is too light and thats why it is diffing out. Try to reduce the roll and that should fix it without hurting performance elsewhere. I am running 2.8mm bars front and rear and it has not reduced traction anywhere on the track. If anything, I have more traction. Keep in mind that a narrow car will roll more than a wider car.(wider weight placement.) Wider cars stay flatter and require a much thinner sway bar setup in order to stay flat enough to bite good. Shortening the camber links and heavier roll bars will actually help reduce roll and increase overall traction. You will reach a point though that it goes the other way so try it and know when to say when.