I think you're severly underestimating the ability of a 6 cylinder motor with forced induction. The R35 GTR can put down more power than the new mustang with a tune and basic bolt ons. My R34 GTR was a 2.6L, ran mid 11's all day and put down 450awhp with standard internals and a few bolt ons.
The six cylinder plan for the new M3 will certainly not be a limiting factor in any form if it's developed properly, and you can sure as hell bet the mustang won't stand up to the m3 in any other category apart from straight line speed. I honestly don't believe it is the intention of the M division to create a 700hp monster, it's all about balance and function as a package.
That being said i sure as hell wouldn't mind an extra 100 - 150hp as standard, or at least them developing a platform that allows more friendly modification (such as the R35 GTR) so the tuning world doesn't have to spend so much money for so little gain like we do now.