My 2p's worth here is that ever since I bought the "new" boat, I thought the prop looked like it was off a smaller engine. Turns out after a long conversation with a helpful Yam dealer that Yam did (do?) what thay call(ed) a "High rake" prop, designed for speed. It's a good 1.5" diameter down on the "standard" G-series. I currently have a "High rake" (i.e smaller diameter) 12.5" pitch on my engine.
The net result of the test run was that I experienced everything mentioned above related to a lower diameter prop. The engine load was such that it was doing 1000 rpm over it's niominal max.
So, having found a willing volunteer to borrow a 13" "standard" diameter prop from (Thanks Al) the next test is to see how much of an RPM drop I get with the "standard" sized diameter. (and also see how the other engine increases with the smaller dia) Theory says I should get about a 100rpm drop due to the pitch increase, but will be interesting to see how much more I get due to the diameter.
What was also interesting was that when I throttled back to the "nominal" max RPM, it was substantially more difficult to break it out. How much of that was due to the engine being throttled so not so much torque at the prop I guess I'll find out at the next test....
I still have a lot of testing to do with regard to prop & engine trim angle (no PTT so need to stop & pull pins etc). If you can wait for a month or so (new trailer on order) I'll get back with some real data.
If anyone has a 15" pitch 10"-ish daimeter prop I could borrow for an afternoon I can give a full report with a wide open throttle......