Everything that I've read about run flats in the performance car world is to stay far, far away. I'm not sure about the actual grip and performance, but ride quality is much worse than their conventional siblings.
My only experience with them was on a 335i, and the ride was definitely not as smooth as I'd experienced in regular-tired 3-series.
I say get regular tires and be done with it. Throw a can of fix-a-flat in your trunk if you're really worried (but then again, the tire shop that replaces that tire will probably hate you for the mess that it makes).
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