There was an article in Auto Express a few months ago which tried non-RFT tyres on cars designed for RFT's.
They generally found that overall handling and grip was below par and warned that non-RFT's should not be used on such cars. The suspension is supposedly set up for the RFT's and not normal tyres.
I think what prompted them to do the test was that a lot of people are now opting for non-RFT's when replacing their tyres due to the price advantage.
I dont know if this was scare tactics encouraged by the tyre manufacturers to get us to spend more money - but it is cause for some concern perhaps.
When my wife had her mini cooper a few years ago we replaced the tyres with non-RFT's and it handled like crap afterwards. It may have been that the replacement tyres (Avon ZZ3's) were just not good to begin with - not sure.
This time we replaced her 120d's tyres with the exact same ones (Bridgestone RE050 RTF's) and have had no such problems.
I am not promoting RFT's but just giving you my personal findings.
I dont know what I will do when it comes to replacing the 19" RFT's on my car, but will need to do more research before i move away from them.
Last: 330cd Sport - Silver Grey, 335d M Sport Coupe, F10 535d M Sport.
Current: AW F21 M135i, Coral Red, Pro Nav, Vis, HK, & Clunky Adaptive Suspension
Current: X5 XDrive 35d M Sport - Black Sapphire, lots of toys.