I would not go to the dealer for tires (any dealer for that matter, not just BMW, but Chevy, Toyota, whatever). They are severely overpriced. When I went in for service they noticed my tires (my Dunlop Z1s) were worn and slipped me a quote with my receipt papers. It was about $400 (like 25-30%) more expensive than DiscountTire or Tirerack for the exact same RFTs, mounted, balanced, out the door. Maybe that was supposed to include a reach around?
Luckily, I have a friend with a mount and balance machine so I order my tires online (usually Tirerack) and pay him in beer.
The open diff on sporty cars is still tragic, but that's another thread and been beaten to death already...