Well, 420bhp was refering to UK metric hp (4500kg-m/min) not the US imperial hp (550ft-lb/sec or 745.5watt), my mistake for not being specific.
But then again, if I was pedantic, I would say that there are also bound to be minor differences in horsepower ratings using fuels of different octanes from different countries, air intake temperatures, and etc... the list goes on.
Needless to say, there is speculation about a future 2nd and 3rd gen V-8 M3 of even greater outputs, towards the 500 hp mark.