I would say you are about right - you are smack in the middle of the recommended WOT range.
I doubt your friend is straining his engine at only a few hundred rpm lower but he will be slightly off the rated 80hp if his WOT rpm does not fall between the recommendations of the manufacturer.
The WOT recommendations are usually the rpm range at which the engine is producing its rated hp any lower and you will have sacrificed some hp.
by my calculations
5700rpm with a 19" prop should give just over 36 knots
5000rpm with a 21" prop should give just under 36 knots
are you sure he has the edge on speed - if so are you sure the boats are equally loaded when doing the comparison and are you sure of the 5000 max rpm on your friends boat.
IMO - from the figures you've given me - 19" sounds the perfect prop.
nothing spooky about it really, its a simple calculation - I put the maths into a program which you can download from a link in the thread below. you will need to have installed the microsoft .net framework v2.0 though to run it.
We must have a very slow hull then, same engine and with supplied prop (unmarked) we got 30knots, at about 5300- 5400rpm, 2 people on bord.
At the 10 hour service had the dealer change the prop now it does 25knots at 5000rpm, same load, this prop is also unmarked and both are white. We were hopeing to get a bit more speed.
Should we change back to the orgininal prop or go smaller than that? The engine has always struggled to pick up from a standing or slow start, before its on the plane. More so with the new one.
)
