New prop info today: First, I re-checked my survey, and my motor at full throttle was 3400 RPM, not 3800. I don't know how I had misread that before. That will make me a lot more conservative in re-pitching my 15"x9" prop. If I adjust it at all, it will be to no more than 10" pitch.
I spoke to Catalina today to ask why they shipped boats with the same 15"x9" 3B props, even as their motors grew from 21 HP to 35 HP. There are several explanations, the most significant of which is that the lower power motors had higher gear reduction ratios on the transmissions. (M35B/Hurth HBW100 has 1.79:1) So the lower power motors were spinning the prop slower. He also thought some of the HPs that I mentioned were continuous while others were peak/intermittent, so that the difference between a M25 and M35B is less than the 70% that someone mentioned previously. Third, he said that they are probably conservative with blade pitch because different people load up their boats differently than others, and as Mainesail reminds us, it's better to be a little under-pitched than over-pitched. A 15"x12" prop might work fine for one person, but might lug the motor for someone who weighs their boat down heavily and/or puts on a super big alternator. He also mentioned three models of Yanmar motors that they had put into the C34, all of which had 15"x12" props, but had gear ratios as high as 2.6.
Soon I'll call Westerbeke and try to see how they set their governors. I'm still curious if my surveyor's observation of 3400 RPM is outside the limit set by the governor.