Kurt's point is a valid one- because the crank throws are offset in an airhead, there is a rocking couple that is significant the larger the displacement. The new R18 does this on purpose, for example. That will contribute to felt vibration
Seems to me that the Motoren-Israel gear is all focused on 1000CC bikes, the theory being that the greatest bang for your buck is to increase displacement to what the Factory did and then take it from there.
Anyway if it's smoothness you want I would consider an equal expenditure on balancing. Here's a good thread on the topic.
https://advrider.com/f/threads/balancing-a-bmw-engine.420346/