There's an yeti type guy on here who runs 305's on the back of his CLS55, including some regular warp speed autobahn runs, and he has no problems. Standard they would be 285, so it seems oversizing can be done, provided there aren't clearance issues.
There's that old chestnut about invalidating insurance as you've moved away from original spec, but when I considered a larger tyre on mine (295 as I couldn't get 285 in Eagle F1's) Aviva weren't interested - might be best to check with your insco.