Your performance with the 115 should be just fine. Many of the 1970 to 1983 20' SeaCrafts were rigged new with 85hp and 90hp V4 Evinrude/Johnson motors, and did just fine. Figure a WOT speed of 38-42, and a cruise of 25-28 depending on prop, rigging, LCOG, trim tabs, etcetera.
And you'll laugh at all the old carb'd 150's and 200's as they fill up at the gas pump
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury