Your facts are fine -but -the engines are going to produce up to 50% more power (not rpm -power) on the tuned , inexpensive exhaust setups .
Your engine is most likely working correctly - BUT- the in cowl setups can not be expected to produce anywhere near the same power as the pipe .
Did you ever run tuned stuf on your DAs's 3W's ?
these engines also respond well but not as dramatic a difference as in the smaller gassers.
When you ge the factory pipe n header - fit it just as it comes and buy a good 18x8-18x6 props such as the Mejzlic lookalike in wood -the Skorepa (ESComposites ) for about a 1/3 the price of a Mejzlic
If you are handy at wood reshape a smooth grained other brand prop to the shape of the Mejzlic - OR try the Zinger Pro 18x6 -it is a good match.
I have tried a number of new smaller gassers from MVVS. Roto/Moki RCS etc. all of them haave a huge power change when fitted with a proper exhaust .
In the USA, the in cowl stuf is widely used but NOT in Europe and you should really expect any power ratings to be with using a mildly tuned setup .or at least a large can mufflers .
That's just the way things are .