You may like to go back and see what others in UK were rated "stock" at dyno. I remember UK crank numbers posted here before adding tunes like Evolve were usually around 360 to 380 bhp.
In another word, your actual few mods maybe did not add that much to stock car since it is obviously underrated from factory. You can't just compare advertised figures with your dyno after installing something, I mean anything. You have to make a "bone stock" dyno first to see "your" baseline. Then you may suggest this sticker added 2 hp etc.
Or I am missing something big time
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"The mark of a great car is one whose overall competence exceeds what you should expect from its individual components and the 1M does just that", Chris Harris.
BMW 1M-SOLD-: TECH: Evolve Race+N55mids, Evolve IC, Michelin PSS, ER cp, aFe filter, CDVx, Vorshlag camber plates, BMS OCC EXTERIOR: trunk spoiler, blacklines, black grills, IND goodies INTERIOR: Alcantara steering wheel, steel pedals, custom mats, MPower e-brake.