1. There's a 34.04mm diameter difference between the two tire sizes. That's roughly 1 1/3".
2. Will it be a big deal? Probably not. Have them mounted, and tell us how it goes. There's a minute possibility that they will rub if you hit a large bump, but there's a lot less of a chance it will happen on the rear, than on the front.
3. A lot of the responses to the OP were ridiculous. As a new forum member, I don't want to see responses like, "OP if you can't afford 2 new tires then you can't afford to keep a bmw." There's no excuse for this level of elitism.
4. OP: Don't be racist. Don't say, "Everybody does it." Don't make excuses as to why you want or don't want to do something. Don't feed the trolls.