I'd recommend people read up on tire racks info about awd and wheel replacement:
http://www.tirerack.com/tires/tirete....jsp?techid=18 Tread difference can cause some serious issues and possibly void a warranty if bad enough. Unfortuntely BMW doesn't really tell us specifics on ranges and how they effect abs and traction control. There is some info around the net but not all models are the same so it's hard to make educated choices if we're risking problems with diffs and safety.
Rears do wear slightly more on rwd and awd but rotating solves this problem. It's also not just acceleration but braking as rear brakes often have more force applied than the front brakes. Fronts get more edge wear from cornering but so can rears depending on driving habbits.
M-sport doesn't have more rwd biast than non M-sport except in very limited cornering conditions under heavy acceleration. All xDrives are the same "80/20" rear biast.
Rarely will manufacturers pro-rate remaining tread life of the other tire (or 3 tires) if you do need to replace 2 or 4 at once, but sometimes you can fight for it or get a discount but stores don't like dealing with these situations. Most tire tread warranties also require all 4 tires to be under 2/32 before a single tire warranty will be considered for cost reduction of new tires, and if you're like most people the tires can wear up to 2mm different which means it's not safe to get all treads down to a warranty amount (but they love this marketing ploy with fine print). You basically have to watch tire pressure weekly, rotate perfectly, not vary driving, and so on to have them all reach 2/32 at the same time which few people ever do.
If you've opted for the performance/summer tire options you're in the worst of everything as the rear tires are wider and can not be rotated. Also, the few manufacturer that do have tread wear warranties are always fully voided if you have wider/staggered wheels on the rear. (and to note, BMW does "recommend" people never rotate tires but most people do for better tire wear) Add this to the fact that some states or companies have rules that new tires must always be installed on the rear if only 2 are being done. I've known two people who went to have rear staggered tires replaced and ended up having to buy 4 new tires when the fronts were only 6 months old and had about 2mm of tread used which I find completely absurd.
I personally was going to do the performance tires until I found out the disadvantages to them and the lifetime cost of tires for this setup was not going to be a good value. I'll be getting a spare for the trunk and switching to non runflat summer and buying additional wheels for a set of winter tires, all with tread warranties 'just in case' and mainly for the ability to rotate all 4 tires. If I have to replace a single tire mid wear, I'll get a new shaved one from tire rack or get it shaved locally.
Edit: Oops, went away and came back to finish this post, didn't mean to duplicate what Bill said...