First, earlwb, thanks for the link to Sceptreflight. Since the Elmore sites deactivated, this looks very good. Host first page comments indicate the Elmore site was source for many of the tests indexed. Regret only that many are reduced to initial page, but understand the reasons given.
1QS2.5r - The (unnamed) author spells out that max power test figures were on 30% nitro. Not scary for US fliers, but rare in UK due to cost . That's an interesting test report!
18K to 20K RPM on the props named is quite good power. Fox's quoted comment that the engine was built strong enough for quite a bit more nitro also fits the reported Max HP value. Remember, too, back about then, Fox also sold BLAST fuel, rumored to be well above 50% nitro. (Someone here may have a "better" number for that.)