The effect of one inch propeller pitch change

How much does one inch propeller pitch change affect motor revolutions per minute ?

If you do some googling some say that increasing propeller pitch by one inch decreases motor rpm 200-400, some say that increasing propeller pitch by two inches has 300-400 rpm effect.
Boat U.S. states that the effect is 200 rpm if the propeller is cupped and the diameter stays the same. Stuart Allen from Dangar Marine also seems to acknowledge this 200 rpm per inch rule.

I wanted to test this in practice, of course every boat and motor combination is unique and the results will vary. I'm talking about Solas Amita 3 11x16 versus Solas Amita 3 11x17. Some say that Solas Amita 3 is a cupped propeller and in this case the only difference should be the pitch size as the diameter is the same (11").

So here are the results:

With Solas Amita 3 11x16 my Selva Madeira 40 XS reached about 5 650 rpm with wide-open throttle. That is above the recommended maximum 5 500 rpm. The motor was trimmed as up as possible.

With Solas Amita 3 11x17 the motor reached about 5 300 rpm. The top speed was two knots lower with this propeller.

Solas Amita 3 labels.

So one inch increase in the propeller pitch resulted to 350 rpm decrease.
I was a little surprised of this result, I was expecting a drop of 200-250 rpm.
It's quite annoying that with higher pitch propeller you don't reach the recommended rpm range and with lower pitch you go over the limit.

This is just speculation but could it be that aluminium is not a good enough material to produce higher pitched propellers in 11" propeller size class. Why does Solas stop the range at 17" pitch ?
It's quite hard to find original Yamaha propellers which have more than 14" pitch.
I'm planning do more testing after I have changed the engine mounting height, currently Selva is mounted to the lowest possible hole and it's producing an excessive rooster tail.
I would like to also test a stainless steel propeller with 16-18" pitch (semi-cleaver type).