The Yanmar has considerably less torque than the Dmax so not a good comparison.
I've seen 19.5 pitch props on the Lsa x45.
Jumping from 17.5 to 20 pitch should give you a few more mph at least.
Not sure how limited in dia the x80 is but I think much over 20" pitch on a 14" ish prop and you won't have enough surface area to take full advantage of the added pitch.
Very interesting awesome project. Keep up the good work.
'06 X2 MCX
"I understand why some people may not want to do this the way I have recommended but I can't understand the death grip some people have on a toilet plunger with a hose fitting." -JimN