I seem to remember someone posting that they had a conversation with one of the engineers back at the Indy show last year and he was told that the widest you could fit on the stock wheels was a 285 due to clearance limitations.
If you want to go wider than 285, you are going to need new wheels with a different offset.
By the way, here is what a 305/50R20 looks like on a 9" wide rim. Too much flex in the sidewall to be any good for handling. 305's would handle better on a 10" or 11" wide rim.
