This is great news. I expect that the 420 to be conservative much like the 300 or the 335i or the 507 of the M5/6 are. They are running hotter cams than the M5/6 based on where the peak power is occuring. Real crank power is probably closer to 440.
No need for underdrive pulleys. Sweet. Not clear from the specs if this effect is added to the power graph. The ability to disconnect the alternator on demand is much nicer than underdriving it.
Engine is lighter than the old M3 engine as predicted here
Depending on how much they beef up the chassis over the 335, the full car can/should be lighter.
The powerband is also much wider than the outgoing M3. Basically an awesome job by BMW.