you are my hero for thinking about this concept. I've wondered about it. Maybe we're both crazy.
I've thought about it as a simple way to go square. Also, according to Tire Rack, my 225 fronts weigh 3 pounds less than my 255 rears, so I've thought about finding a cheap pair of either 8 or 8.5 wheels, going square, and downsizing tread.
I doubt I'll notice less tread in the back in a daily driver 328i and maybe with a square setup like this I get more neutral steering and some other benefits from less un-sprung weight.