Unless your alloys are RFT specific then you can't fit runflats. The other way around is possible where you fit ordinary Tyres to RFT alloys but not vice versa.
I had run flats on my 4 Gran Coupe, the road noise was horrible.
The Z4 generates less road noise compared to the gran coupe, I get a little more noise from other vehicles now (not that much), but its less annoying then the sound of run flat tires
I feel like run flat tires are also a lot harder compared to normal tires, not that it bothered me that much, but the difference now is very noticeable even with the harder suspension the shock absorption seems a lot better
I also only had one flat tire in my life (I will most likely get 4 flat tires tomorrow now). So the discomfort is not worth it to me. If I do get a flat tire there's BMW assistance.
I also think if you want to go from non run flat to run flat tires you have to change the rims.