The specified 4-6 is plenty of hp. You're not going to plane in that boat, you're going to go at hull speed
at most. Furthermore, it's a sailboat, so count on going in a relaxed manner.
If you want to go in the manner you're describing, get a hot skiff and fight with it.
I had a 19' O'Day Mariner with a 5 hp outboard
. It got up to hull speed
with power to spare. I don't remember the mpg. I have in mind 1 mpg, but that can't be right.
Now I must be the last person left on Earth who prefers a smaller motor
. They're compact easy to handle, take off and install for maintenance
. I am capable of putting a 3.5 hp motor
on the Catalina