Re: Drilling out the exhaust.
1st problem, noise ordinances, get loud, get noticed, get ticketed.
2nd problem, IF, (notice that's big), IF, you did manage to get more air to flow, that would change the fuel/air mixture and melt pistons. It ain't so easy to jet an outboard, but it can be done, just be ready to rebuilt it if/when you smoke it til you get it right.
The extreme low pressure area created behind the prop hub by forward movement in the water works VERY well in scavaging/pulling the exhaust out of the motor.
It is VERY difficult to improve on Merc engineering, they have been doing this for quite a while.