Hi all!
The germans on http://www.g4rce.net says that 33x12,5 on 15" rims is no problem without mods. I've seen it myself, looked excelent! But exceeding this, you are facing lifts and mods!
Good luck!

I've often thought of upgrading (from 31x10.50) to 33x12.50, but then I remember one thing -- my G wouldn't fit in my garage. If one has an ordinary garage, the standard tire'd G barely fits in the door -- at least on my garage, the door allows about 1" of clearance at the roof. With the 2" taller tires, that will result in 1" greater (one radius) height -- and swallow up any margin of error that my door (and it's springs and motor) might need.

A related aside: I've heard that the issue of garage door height (and prospective buyers concerned about their G fitting in the garage) has reached the designers -- and that the next generation G (the so-called MLG) will have a height control (like the Lexus LX-450) to make the G stoop down (as well as rise over trail obstacles) and fit through most garage door openings. Now if their owners would just remember to use it. :-) .

One will only have to do that once or twice before your insurance company gives you a cancelation notice. When I move, I actually have the height measurement with me to a assess the "new" home's garage height. Most homes (not necessarliy townhouses) built in the last 20 years are okay.