If wanting to beat that old 20 HP, buy a induction Hardline tach, are cheap. Don't play with less pitch props blindly as OB could be over revving for that particular load and not knowing about it, much worse if lightly loaded. Don't hallucinate with max speed achieved, wot revs is all...
If OB sits at the Tip Top transom spot while correctly trimmed, lower leg will have the least water drag, if rising OB passing sweet spot prop will aerate on choppy, windy water cond and at tight close turns at speed due to poor water bed height for prop to grip top, best forward prop thrust is lost.
Happy Boating