Two problems:
first --there is no perfect length for tuned header pipes
secondly - you did not address which props you use and what rpm range to are using.
The correct length is made up from the rpm band desired and how easy it is for the engine to get there .
It is not uncommon to get incomplete advice -or understand what is going on.
Your problem - most likely is you are using too much prop load to allow th engine to match the setup.
There is a lot of plain old Bull sh it afloat about how certain setup can produce power increases and how the engin can be induced to pul bigger props .
In a nutshell -most setups which are tuned - raise the torque - or move the best torque further up the rpm curve.
Torque is increased if the engine will pull the same prop faster --than it did with NO tuned system.
Here comes the rub-----
When an engine produces more power -especially from a tuned system --it needs more fuel to do this
So
In order to do that -the carb must be set richer ----which makes the upper rpm band speed --harder to reach
Once the engine reachs resonant speed - the torque INCREASES and that takes more fuel.
So, how to fix that problem is the question
1-- you simply reduce prop load, allowing the engine to more easily reach into the "resonant" rpm.
2-- or you increase the header length and if the total pipe/header combo is right - the engine now moves up with little stumbling.
There is no free lunch
to pull more prop load - the pipe length may need to be quite long
the rpm gainwill not be as great but the rpm and resonant length will work
A lighter model will unload the prop more easily than a heavy draggy one - so a setup which worked on one plane may be no good on another
On your engine, the DA150 -- maximum power on th e cans/pipes will be in the 6000 rpm band -well into it - NOT under 6000
a 30x10 Mejzlic will likely give you the best power -easiest transition
Cramming on more prop load will just make it all worse