That rubbish your duyo operator told you is just that!
If he had any idea he would have logged your boost pressure to wheel speed and engine rpm, like this...........
or
or
Then he could pass on a nugget of information like he did lol......... based off some facts! not some bullshit comment based off his own ignorance and lack of basic knowledge!
I have NEVER EVER seen any rotary manifold exhaust or intake on a turbo based motor cause that effect!.... I can only provide you with PROOF! not some bullshit excuse like he told you.
On a positive note what I would suggest to you to narrow down the root cause is do some basic mapping of things like intake boost pressure trace and air fuel ratio to further narrow down root causes of your particular situation/problem.
My quick opinion is it is due to the fact of your WM system and poor calibration to suit it.