Dunno if this helps but we bought new Yamaha 90 last year with 13.25 x 17 prop and couldn't get over 4100 rpm (WOT range is supposed to be 4500-5500). We dropped down to 13.5 x 15 and get 5100-5200 easily and if it's really flat 5400. We also have 13.75 x 13 which gives great acceleration for skiers/watertoys but top end speed is poor. So the 15" pitch seem the best compromise. I'd say you want to get more than half-way into the WOT range but not worry if you don't hit the max.