If your profile is correct and you have an 06 v8, you have some extra room in the wheel wells. The TDI version of the T1 has intercooler vents that take up some extra room (later models - 2011 and newer have flattened them out).Upgrading from 18s to 20s
Does anyone have 275/45/20 instead of 275/40/20s or are they to big?
If you do go bigger make sure you test fit them and then turn them full lock each way to ensure they don't rub. If you have air suspension, make sure to drop it down to Sport or even Load level while doing so.
Also - a note on 20's.... I've got 275/45/20 summers and 255/55/18 snows. There is a LARGE price difference between a 20 and an 18. Also, there are more options in 18 than 20. 20 tends to be more of an on-road size, whereas there are some pretty aggressive off-road tires in 18.
Hope that helps! Good luck!