OK, here are the final results:
Lube MVVS oil (mobil1, red, 1:25), Fuel 98 RON (premium 93 PON), Prop 24x10x3 Mejzlik. Exhaust Header 14”, outer dia 28mm.
TD 75KK, shortened, 6390, 7.7hp
TD75K, 6460, 8.0 hp
3204 mvvs,6350, 7.6 hp
TD110 6450, 7.9 hp
TD75,6420, 7.8 hp
MTW T3 tuned pipe 7000 rising to 7100, 11.0 hp

MVVS reported 10 hp on their new design canister muffler, still in testing phase. JANCH has one of the prototypes on his plane. Maybe he can comment on final rpm.

The fact that the TD75K turns faster than the TD75 leads to the assumption that the header pipe is too long for these mufflers (the TD has a longer inner section of pipe as compared to TD-K)
The TD75KK is shortened to even shorter total length to try how it would run. Penalty in power is not too high, and the muffler has excellent muffling and power response. First chamber volume (403cc) is only 5x engine displacement. No signs of sagging in power, so thermal load on the engine is OK.