An engine is an engine and anytime you free it up you gain power so I would say yes but remember you have to modify both ends of the system, you can't do just the exhaust wihtout doing something the carb/injectors.
To get more power out you have to put more gas in.
Not a dumb question . . . yes more power as the flow of the exhaust will be better and less power taken away from the engine.
It is difficult to say how much power, and as stated, you will be limited by what is going into the engine.
The more popular name is "thru hull" exhaust. When you do that, you need to address noise suppression, unless you want to join the go-fast, be-loud (GFBL) part of boating.
Don't forget that there is some measure of exhaust extraction with a thru prop hub system. I would say that unless you are going to race mode like the go-fast guys it is probably a wash and wasted money.
I've helped a couple of guys install choice exhaust systems, and have ridden in those boats. I can't say that ANY of them have had any change in speed/hp. If anything, the thru-prop might actually flow easier, there is basically a giant vacuum sucking the exhaust out!
Smokingcrater is correct. As the boat moves thru the water, the exhaust is sucked thru the prop. This efficiency is as good as wide open pipes and much easier on the ears. As boats with open pipes pass my house at idle, the noise is deafening. I have developed a deep hatred of those boats and their owners.
I've seen many dyno tests on race engines for cars and most will perform with more power going through the right muffler when compared with running a straight pipe after the header while experiencing a 10-20 dB drop in noise level. I have yet to see a sport boat that had a performance improvement by making the exhaust loud. The owners of the boats sometimes think their boat goes faster because it now 'sounds powerful' but that is not speed. Bondo's assessment is correct on actual marine headers helping but you could still flow through the prop with those installed. Usually you need to match other changes to increase air flow to realize much power increase.
