[KTMasean]

If riding with sense, warming the engine, shifting high up in the gears and using quality maintenance parts & oil (with the correct specs, nothing stands in the way of more BHP. Take 2 strokes for example.

 

[KTMasean]

A German company (former KTM dealer) Pirate Racing, fitted an in-house made ram-air pipe with BMC cone filter, a PCV and some fine tuning getting an RC390 up to 52 BHP.
Unfortunately the discontinued their ram-air pipes that were made from a plastic and had a low resistance coating so now the only one I know of doing something similar is Chimera but out of aluminium and no coating and not really nicely done.
You can get these from 1Finger US and they offer a K&N cone air filter.

 

[KTMasean]

There are turbo charged Dukes with extended swing arms to not flip over. No idea what they do on your 0 to 60 time though, to me a senseless exercise.