I considered this very thing, but then I saw it on a car and decided against it I really dont care for it.
In order to keep your speedo accurate you would have to do one of two things, first you could reprogram the computer with a tuner, if this requires going to a shop it seems impractical and far to costly. Second you could avoid the shop and get lower profile tires for the bigger wheels which would be in the rear...but you talk about retarded, put a taller tire on the front of a rear drive car...that looks terrible, especially the bigger the wheel.
I recommend doing all one size and keeping it simple and cost effective. The looks are NOT worth the extra money, especially when it looks BAD!!!