What about heating problems for an M turbo.
Go drive youre 135i or 335i on track you will see the temps go thru the roof.
Or the next one.
An M is perfect to balance on throttle thru corners try that with a 135i or 335i. with a tuned one it s even worse.
And try do nice slide balanced on throttle with a turbo. Goodluck for that. You proberly end up in the hedges.
The best racing engines are always the non turbo ones. For the straights the turbo s are the fastest. But for nice cornering balancing on the grip with youre throttle is gone with turbo s. It s either to much or to less.
I say stick with non turbo. I don t see it happen that a turbo engine has the throttle response of qn M engine we have today.