Get bigger tires when you get bigger wheels. 15" are lame on MK4s. Look at craigslist or vwvortex wheel classified for some decent 17s or 18s. You'll need 5x100 with ~30-38 offset. If you plan on lowering the car then get 30-35 offset wheels.
Thanks, that helps a lot. My tires are pretty new, so for now I guess I'll put my money into coilovers and whatever else I need, then I'll get new tires and wheels when the time comes. You said to go 17" or 18"... do you recommend one over the other?
i personally think 18s look better. but 17s will be a slightly less harsh ride. Theyre also easier to fit when you lower you car, as in you have to lower it more with 17s to tuck. 18s would be easier to fit if you arent going crazy low.
I don't plan on taking it too low, I mainly want to kill my wheel gap and then I'll see if I want to take it any lower. For now, I'll just do that, then when I get my new wheels, I'll probably want it lower. (Aka slight camber.)