21 Superboat w 250ProXS

Happy Thanksgiving to all!! Trying to dial in a 1985 Superboat with a Mercury 250ProXS (two stroke) running a Tempest Plus 24, 20in motor, 14inch setback, no jackplate, shaft is 1 1/12 below the pad. Running 6100 at 72mph, prop is strong out of the hole and strong midrange, seems like 13% slip...does this sound normal or what else can I do to grab a few more mph, I will try a 26p Tempest Plus in a few days. Thanks and enjoy!!

Trophy plus 24 or26. Mine ran 83.5 with a 250 efi