Lovely looking car, welcome.
I ran runflats on my E86 on OE 18" wheels for years, but recently switched over to non-runflats when I changed my wheels to a different OE 18" style of wheel. In my opinion, this resulted in a slight improvement in ride and I haven't noticed anything significant in terms of handling. I enjoy driving my car but I don't regularly explore the limits of its grip or track it. However, I went from good condition Michelin Pilot Sport 2 Zero Pressure to new Michelin Pilot Sport 4. I have a distant memory that the Bridgestone runflats that my coupe came with were not good. I would say it probably depends on the brand of runflats you have and how worn they are.