Looks like you found the same problem, but came up with a different solution.

I picked up a Zinger 24x10 and tested it yesterday. I'm getting an additional 600 RPM out of it vs. the TopFlite 24x10. The Zinger is not as think, front to back, has a narrower blade tip and appears to flex much less. So, now I'm at 5600 RPM, but still short of the spec in the plans which indicate I should be getting 6000 RPM.

Starting to wonder if the spec is wrong....

Others have suggested trying a Zinger Pro or a Menz, or going to a smaller prop. I have a 22x10 that puts out 7000 RPM - would like to find a source for a 23x10.