In Targa we adjuested the rear wing on our R33 (from 3rd most downforce to 2nd most downforce), we couldn't belive the diference it made, the back stopped dancing under heavy breaking, and it stopped wanting to drift round sweepers.
Our car also has a flat carbon tray under the front (N1), and is ment to have a carbon diffuser on the back, but apparenty there often flogged on import, and Nismo want $4000+ for a new one, i'd be interested to see what extra difernce the rear diffuser did.