I only have a lowly 335i and I still always run that on V-Power. Runs smoother, feels like it pulls harder and I get a slight increase in mpg which offsets the few pence per litre difference in price. If I changed to an M3 I wouldn't use anything less to be honest either.
Any high performance engine is susceptible to det when pushing on, yes modern ECU's can retard the timing to avoid problems but then you loose power and the possibilty of causing damage to the engine is still there. This is a car which will then no doubt be sold on as "one careful owner"
Why would you spend thousands on a car like an M3 to then scrimp what amounts to a few quid a week on petrol? Also what else will the owner be scrimping on?
Sorry but questions like this about premium high perfomance cars completely baffle me??