So I got a reply back from the company.
Thank you for contacting Noelle Motors in Germany. I look after enquiries from international export clients.
To answer your question, our M3 conversion does not change the capacity of the engine, so I website should read 4,0 and not 4,2.
Secondly, regarding warrenty it depends on your local market. From experience, changing absolutely anything on you BMW vehicle can void your warrenty. So if you are worried about this, we advise to leave your vehicle standard..!!
Most of our clients of course ignore this and take the risk...
You will see that we offer a 335 flash, which is undetecable by your local dealer and cannot be overwritten by future BMW software updates...
So it looks like the 4.2L on the website was just an error.