My assumption has always been that the stiffer sidewalls of the RFTs means they can reduce the stiffness (compression/rebound?) of the suspension by a notch to compensate. That would then suggest that with non-RFTs fitted (on a car 'tuned' for RFTs) the overall suspension setup would be a tad too 'soft'.
Personally I am a wee bit skeptical. Consider a 320i SE for example. That comes on RFT-shod 17" 393's, yet you can spec it with 19" 439's on the configurator, also with RFTs. My guess is that the 19" wheels would give less compression and rebound than 17" ones, yet despite being so obsessive about the handling claims of their cars, I don't suppose for one minute that BMW alters the suspension settings in any way depending on what size wheels you go for. I could see how the adaptive suspension might compensate, but the stock setup no way. Also, I'd be surprised if the ED which has non-RFTs had a different suspension setup to say an equivalent SE, Modern etc., which does ride on RFTs.
So for BMW to start claiming that fitting non-RFTs would affect your warranty, or insurance companies suggesting that somehow a non-RFT equipped car might be a higher risk - I think they're just milking it
But then I'm looking at this from a layman's perspective and am happy for someone who knows a thing or two about suspension to put me right