There's been lots of research by GenVibers re wheel sizes and weights for our toys, as you are hearing, but AFAIK, per this thread I'm the King of Wide!:
http://forums.genvibe.com/zerothread?id=4132I have 275/40 x 17s on 10" rims on my Vibe. They did scuff on heavy bumps when combined with a 2" drop, but with 1", there's only occasional light contact. That said, I certainly wouldn't go this far on a serious autocrosser. A 245 would likely be OK, but a 235 would be a safer choice. I adjusted the effective offset for only ~ 3/8" clearance to struts, inner fenders, etc. and have never had contact there.Although experts usually recommend keeping the diameter within 2 -3% of stock, a number of racers I know use much shorter combinations just for the track, to shorten the gearing and further lower the car.Good luck!