I don't think the real numbers would be leaked so easily, it's a conservative est. it seems and surely BMW is not crazy enough to think 4.3 to 60 is good enough as a marked improvement from the last M3 (not 0-60 is the end all, be all...). They should know very well, the competition for this car is well into the sub 4s already with sub 12 1/4 mile times, and to keep up they'll have to at least match that number + be more than average on the track.
Like all past Ms, it's supposed to deliver close to 911 S figures but at a lower price point.
(i.e.RT clocks the 991 Carrera S - 3.5 s to 60 and 11.8 1/4 mile). Then there's also the matter of the new Corvette Z51 pack that is darn close to the M3 price point with impressive numbers.