"Therefore, I was strongly advised to stick to runflats summer and winter tires as mounting non-runflats would alter the feel and behavior of the car to the point of affecting safety."
Based on that statement, BMW should not offer the following package at all:
Cooling and High Performance Tire Package
+$1,850
20" M V-spoke jet black wheels, style 728M with high performance non run-flat tires
Else there would be TWO versions of each model - one made for runflat tires one for standard tires..... that is nonsense. Yes the feel will be altered but in a better way - smoother rides = higher comfort and not feeling every little uneven part of a road let alone holes or other major road defect. Should I add less weight? And and and and. Now that rep should explain how would this alter safety other than remaining on the road without a spare tire and having to call for help? Indeed it is not an "ideal" situation should it occur, but it does not make the car unsafe by any means! Nor does it damage or compromise any of its components.
Plenty can be read and researched regarding runflat vs. non-runflat and one should decide what makes most sense for their driving needs. Fact is most people wish for non-runflat and avoid runflat tires in the 1st place for obvious reasons. Thus do not worry and mount if you please some nice sports NON-runflat tires as the ones offered in the high performance tire package.
20" wheels are just perfect for this car and its body chassis. Actually I am even leaning towards a nice 21" setup but not a priority of mine and I am first intending to perform a few more upgrades. 19" for winter is perfect, as anything below that makes for a strange stance in my opinion especially when looking at the car from the back.