Brrrrr! McDonald Beach looks chilly.
Boat runs nice. 65 mph in a boat that big is pretty darned good I think. what length is it? Looks like you've got just a bit of porpoise on the big end.
Some others won't agree, but your 200 rpm gain is pretty typical of the change over to headers that I've seen/done. I know several guys that made the switch and gained the same rpm without doing anything else.
As for your speed and rpm vs impeller size, that would depend largely on what rpm your maximum horsepower is coming in at.
Although you may gain a little bit by going to a smaller impeller (higher rpm), I don't think it would be worth messing with. I'd probably vote to leave it as is. 65 mph at 4800 rpm is excellent. You're not pounding on the motor and the speed is very respectable. In all liklihood you would probably lose some of the hole shot by going smaller impeller-wise too.