Back in about 1994 a guy in Norway went to considerable trouble calibrating various props and finding out how much HP was needed to drive them at different revs. APC were the main ones used as they had little variation in any given size. So basically all I did was look at the graph given in the April 1994 issue of Aeromodeller magazine. So now I can say that your OS has 1.37HP at 16.8K revs

I'm glad you mentioned the error in the load formula because I came up with one that seemed to work but didn't make sense when compared to the Diam^3*pitch so now I'll use the revised one for load figures.
10x4 load = 40,000
11x4 load = 58,564
10x6 load = 60,000
You can see that now the load figure for the 11x4 is much closer to the 10x6 than with the original (^3) formula.

OK, now this is what I did (and it's only approximate because you have to assume the same HP driving the prop). For starters I'll do the easy one first which is comparing the 10x4 with the 10x6. And I'll use your 16.8K revs this time too.

Divide the 40,000 by 60,000 which gives 2/3 or .666..... then find (.666)^-4 which gives .903 then multiply by the original revs of 16.8K to get 15.18K (near enough) which just happens to be almost exactly what the graph for an APC 10x6 shows at 1.37HP

Doing the same maths for the 11x4 prop's load figure gives .909*16.8K = 15.27K revs.
The ^-4 term is just finding the square root twice and that's where I couldn't figure why it worked with the ^3 load formula so I just didn't worry about it because I was coming up with the right numbers

Interestingly an OS site claims 1.2 HP @ 18K so maybe the Mousse can is working. But these formulas have to be used with some care because of different brands of prop varying in the HP needed to drive them at the same revs and changing the revs of the engine changes the HP available anyway. I'd take a guess the actual revs you might get with the larger prop would be a couple of hundred less than the calculated figures because there's less HP to drive it.