@Uli_HH: I really am not sure about the "new" engine. How long does it take. They should already get something concrete. Look at the M6 prototypes, they started testing long before the M5 Concept was even revealed.
Concerning the S54 engine, they cannot use just so. S54 alone is already heavier than S65, and add the heavy weight of the complexe turbo system, it will weigh more than S63, which I think but have no fact, is heavier than S85. S54 block is made of iron. Would it be possible to use the same molds but pour magnesium/aluminium? Also, S54 is a high compression engine. The ones that were running turbos had moddified internals.
Insider SCOTT said that the M3 will get a convertional 6 cylinder engine, by this I understand I6, which would be N54 by M. He said for the M4, however, a completely new concept will make its way. By this I would say M3 F80 will get the 3,2l I6 based on N54, and the M4 F82 will get the 3,3l V6 based on S63, they'll have more time to develop it properly. But then regarding the problem of high costs, it does not seem very plausible.