Don't believe a word the dealer tells you.
.....their main goal is to sell you a $3500 Winter tire package.
Sure, you are going to loose performance with Winter tires (don't get true performance snows). It's winter. Think Safety and Black Ice.
With these run flat cars, I still put regular snows on.
Buy a canadiantire patch kit, can of tire inflate, and a portable pump and you are good.
I've had to use it once, and never missed not having a spare tire (nor run flats).
..... didn't even remove the tire.
TireRack has a Firestone Winterforce 17" on Alloys for $992/set
I'd run a 225/55/17 for snows (all four - thinner is better)
You culd run 245/50/17 on the bigger 5 Series.
You could also run 225/50/18 tires.
I usually don't install the TPMS either for the winter setup ..........Your call.