I seem to recall someone saying they were running 8.5" of thick black rubber - on 19" alloys. Its certainly been discussed on here several times. Maybe another candidate for the FAQ?
Got any pictures? I've just replaced my rears with std 235/40s but maybe next time I might go wider, it's only £20 per tyre extra. I guess it'd make little different to how it drives since it's only 1cm extra each side in width.