trimmed up on calm water the 27cc does around 40mph. i have had my Chesapeake to just over 39 with her trimmed out and running. The cc is slightly faster due to weight
since you have a merc motor. I would look at the Merc Rev 4 prop. It has a ton of bite too. See what the merc prop selector states you should have by entering your current performance:
http://appcenter.mercurymarine.com/propSelector/home.do
Good luck!