I recently got some new custom made ADV.1 wheels for my italian beauty, 20x9 Front with 255/35-20 Michelin PSS and 20x10.5 rears Originally I put 305/35-20 Michelin PSS in the rear to give it a nice wide meat stance and to fill the wheel gap, this accomplished that however the speedometer was off by 5.5% below the stock reading and the edges bulged too much for my liking so I switched to a 275/35-20 which is only 2.5% off from the stock, this looks better and is closer to stock and is 6lbs lighter per wheel. However the 305/35-20s did not rub and were 1.5 inches larger in overall diameter and 1 inch wider so you will be fine if you run 305s. I may switch to 295/30's or 35's in the future. I almost wish I would have gotten 11inch wide rears so I could run 315s or even 325s in the rear, how great that would look, and yes they will fit, I measured.
If you have any questions, I'm around...
Rain pic is with 305/35-20, Underground Garage pic is with 275/35-20 and the outside my garage pic is with the 305/35-20.