I would find a Discount Tire store (they're everywhere) and pick up a set of Michelin Pilot Supersports. I even got a few hundred for my Goodyear runflats. Out the door I was at $1050. for 19" tires with much better handling and performing tires, especially in wet. You also have the option of a road hazard guarantee. they are also lighter than any of the runflats BMW uses. You should definitely notice the difference.
I've also got a small air compressor, patch kit and aerosol puncture sealer in my trunk, taking up much less space than a spare of any sort would. Did the same thing for my Lexus SC430 and never had a flat in 5 years.
550ix, Dark Graphite Metallic II, M-package, Dinan Stage 2, 20" wheels, conven pkge, cold weather pkge, Prem Pkge 2, Paddle Shifters, vent seat pkge, sport transm pkge, fold-dn rear seat, etc. Black Dakota Leather, Light Wood trim