Different size rims
I think it would look awful . You will most likely end up with more sidewall on the front than the back (which will look weird IMO) , unless the rear tire diameter is very large. Actually , the opposite (17 on front , 15 on rear )wouldn't look too bad. Pretty common to see late model cars at the drags with smaller diameter wheels on the back. Here's a cool RR pic I found on the net :