well over here in the uk the hyper 8 pro has always been doing excellent and i was talking to a view CML team drivers all of which use the hyper 8.5 and they all love "hyper 8.5 wins the 2007 Euro's" it basically the difference is front and rear bigbore shocks the new fr2 chasis and the new chasis braces new radio box. but this does cost quite a bit for those parts.
personally i would go the extra mile and get the 8.5