I can't see how a bigger engine will help at the harbour entrance in a displacement craft. If max hull speed is 6 knots then with 3 knots of tide your sog will be 3, but you are doing 6 and as you say, a bigger engine wont give you more.
Calculator for speed: http://www.psychosnail.com/boatspeedcalculator.aspx
or http://www.cncphotoalbum.com/technical/hullspeed/hullspeed.htm
I guess the guys at PYC should be able to offer advise for the best engine for your requirements.
Rob