Your results are smack on the money with what we see here .
So -- My EFYAK-runs a ZDZ50 with AMT pipe and turns a 22x8 Mejzlic at an honest 7750
also a 23x8 Mejzlic wood clone (Skorepa) at over 7200.
This gives serious instant recovery
My modified 80 single on tuned pipe runs -- a real 7000+ on MenzS 26x10-
again serious power
If I stick a small muffler on either one - power become just adequate.
A24x8 will load the ZDZ down into low 6000 and there goes the max torque -same as happens with the DA50.
these smaller props have to spin to work .
When we did an article on a Funtana 90 -we used a Roto25 and a 17x6 prop -this worked but an identical model with a OS 1.2 -seemed underpowered -so we did static thrust tests - the OS on a 15x6-produced 10 lbs thrust - The Roto was under 13 lbs
You simply can't get enough power and keep the weight down -at these altitudes -to cause a "overpowered"problem.