I agree with concensous on ill advised widening cookie cutters for competition wheels. Good Year guy I spoke with said 8" wheel would creat a little more contact patch but no concerns about safety, etc.
I used custom made 7.5" wheels that weighed less than 10lbs each. Turn in on slightly wider wheel felt a little better to me. But I'd focus more on reduced weight than width. I could feel performance difference between my Jongblode wheels and the much heavier cookie cutters in acceleration.
You know once I went to those lighter and wider wheels and differnt exhaust my car started getting very close to TTOD at AX events until the LSD crapped out.
Of course we all know canti's on 8" wheels will explode and make you a danger to yourself and others.
I'm running 15x7's on much heavier Boxster with Canti's and car handles even better than 914. It just doesnt have same power to weight ratio. but no excessive flex etc.