My guess is that the engine will be an N55-block but with two turbos, exactly like the Alpina did with its new B3. There is no way to control lag in an M-worthy manner with only one turbo at the 350+hp power level (would need quite large turbo).
The S55 engine block is too tuneable (closed deck) and would put M3/4 sales in jeopardy.
Also I think that the weight savings will be substantial (BMW can produce carbon fiber cheap with its new CFRP factories) but the power not much higher than the 1M (max. 340-350hp).
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