Hello
About time I update this thread.
I changed to a penta 8,1 GI 375 hp.
With F9 props, the old where F6.
Now Max speed is 65mph at 4600 rpm
And it is much more quiet at all speeds.
I did install karnag's soundabsorbing material.

Bigger motor will probably have too much backpressure running thru prop, especially at 500+ HP. Best bet is to go thru-hull and utilize a set of turn-downs to get the exhaust to exit below the water line. As mentioned, maybe add some additional insulation in the engine compartment to lessen mechanical noise. But make sure the engine can still get enough air.