I haven't personally pushed the 18" envelope but have 275/40/20 all around so I can rotate them. You can probably go a bit bigger in back.

If you want to go 17" the bolt pattern is 5x115 (offset 25mm to roughly 19mm)

17" would be better for the drag track and give you more options on tire sizes that will affect gearing ratio.

 

My concern with going down to a 17" wheel is this, will the wheels leave enough clearance for the brakes? If so finding 17" wheel with that bolt pattern and getting the backspacing right will be a breeze. I'd go up in size however that would bump me into "bling" catagory.

 

Not that I have done this but from what I've heard the 17" wheel will clear the breaks.