Wider tyres on the rear sound like wheel diamater issue to me as well as good way to promote understeer.

Other cars of similar performance work nicely with 245 18's, you would need massive power to need to go even wider I think. Our Subaru race car runs 24/64/18 slicks and it doesn't have any traction issues with 530bhp@1070kg (it overheats R888's very quickly though when street legal tyre is required).