I ended up taking off the rear wide tires (which are for sale on craigs boston right now), and then I went with OEM BMW style 168 wheels (9.5" wide) all around. So the rear foot print is only an inch narrower.

I did not need to get spacers for the rear. As said above... with the offset of the front wheels being less than the staggered rears, it only ended up bringing the inside of the new rear set-up in 2mm (which is pretty minimal).

I ended up mounting Yokohama Spec-X all-season tires (275/40/20) on those rims too instead of the michelin all-seasons. The Yokohamas were only $600. The michelins were at least twice the price. Note that mounting and balancing these at a certified shop like Sullivan Tire costs $300.

But, I am getting slightly better gas mileage, I sold off my winter wheels and hopefully the rear fatties, the ride is actually A LOT better with the Spec-X's than with the Diamaris, I won't have to do a winter wheel change over (which usually costs about $100/year), and replacement costs for the Spec-X tires or even the 275/40/20 tires will be reasonable down the road.

MAJOR PLUS: ROAD NOISE is very minimal with the new set-up. The Diamaris were SOOOO LOUD.

Also note, the steering is a little looser in the new set-up; so the X5 seems a bit more agile, but then again probably less stable due to the non-staggered set-up...could be the 420 treadware of the new tires or simply the fact that they are new.

So, I would recommend doing this if you want a smoother ride, no hassle with wheel change over, lower replacement costs down the road, better gas mileage, longer tire life, and less road noise.