Dom, FWIW, I don't have this issue, I have standard suspension (removed M-Sport as I didn't want it to be too hard)... which would be same as yours.
You do feel / hear the tyres on the rough stuff, but only the normal sounds you'd expect with 275/18's RFT's at the back. Sounds like you need some adjustments !
Alpine White (
) F10 520d M Sport Auto - 8 speed Auto/paddles - 350M 18" Dunlop Rear/Michelin Front RFT's - Deleted Sports Suspension