If I get, say 15K on that 6x3, I would say it has enough power. I was going to run it yesterday, but my test stand won't accept the engine. I need to radial mount it on something.
I was able to run a Cub .099. After a huge amount of evil black crud cleared out, it ended up pretty good. Turned a 6x5 power prop at 11K. Even with whatever was grinding itself to bits inside, it finally ran with a clear exhaust and still seems to have good fits. I was running 10% Nitro, and suspect that going to 30% would make a huge difference (Cubs have rather low compression designed in, so they can handle the Nitro well) I have 3 more of these and plan to run them all and compare performance.
This was the engine specified for the Wee Duper Zilch. I have my doubts that it would work very well. I want to build that model for OTS and Leprechaun, but will power it with a PAW .061. That engine should turn a 7x5 somewhere around 12K.