The worries about replacing runflats, or even the need for a spare, are exaggerated by the industry. Road debris (and maybe chuckholes) is the modern worry, not tire failures. Our family has nearly always recovered the annual costs ($53-85/yr) of AAA from services delivered for maps, battery jump starts, gas, or towing. I haven't changed a tire along the road in years because we've had no flats.
We use dedicated sets of wheels for each intended use. This has worked very
well. 15x5.5" R81s for the weekday commute (sidewall compliance gives a smooth ride and better fuel mileage), 16x6.5"/Falken for huge grip in track days, and we've even got a dedicated set of mounted directional snow tires (on R81s) which turn our MINI into a tractor in the snow, and working just fine in the mostly wet commute here!
I don't know who at BMW said not to use regular tires since the Cooper's standard 15 in. wheel comes with a non-runflat tire. That is the setup to use for the best ride, and it performs quite well as it is also the lightest combo.
dukesantos is quite correct here. It's important to add that the Cooper carries a full-size spare under the boot.
Also, with very carefully choosen
tires, I'd be surprised if any of the standard Cooper S wheel/tire combinations would accelerate more quickly than the R81 15" wheel on the MCS; no question they're faster in the Cooper. As dukesantos says, it's the lightest combo (and a well-kept secret?).