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!

Just a caution if you're going to 33" tires
 

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.