I do not think the MTW R2 is the culprit. Something else happened along the way.
The pipe is develloped on a DA50, together with DA.
I am currently testing a MVVS 58 with the R2 tuned pipe. The engine previously had run with canisters, but it is still not run in.
Header length 40 cm to start with. running 3W prop 24x10.
No tweaking of needles yet; It started up right away, idle and throttle response rather OK, idle seems rich.
played around with short full throttle bursts for about half a quart.
Refueled with 2.5% oil mix
Started up again, warmed up for a minute, and went to full throttle for rpm measurements. (6150), but engine sagging. returned to idle immediately and stopped engine after a minute.
closed idle needle 1/4 turn, and opened up main needle half a turn.
Idle, and throttle response OK. Warmed up, went to top rpm. 6650, still sagging, so went down to fast idle again.
Subsequent throttle-ups all showed 6650 rpm. I did not wait for the sagging before going back down to fast idle. the engine clearly needs extra running time with the increased power level.
This is to show, that very little tweeking is needed. My engine however is larger, which makes the setup more critical. Your DA should tune sweeter.

Typos edited