I think I will get the engine re-tuned/re-mapped. Will let the engine wear in for a few thousand miles and then get it done.
I purposely went for the 420 rather than the 428 as the re-map will take it up to and perhaps past the 428 performance for a fraction of the cost.
The guy from DMS gave me the below stats:
The power increases from 184bhp up to 275bhp with the torque
climbing from 270NM to an astounding 420NM, a total transformation, more
torque than the 335i.
Am a bit concerned about warranty implications though so will have to think it through carefully.
Not planning any external mods at this point but never say never!