It's very rare to see any performance related "upgrades" to 4 stroke motors under 150hp. VERY rare.
Yes I've seen a turbocharged (DIY, and UGLY) 4 stroke 25hp Yamaha. Nitrous? Yes I've seen that too. These duck hunters are crazy. Used to be, they'd have duck boat races over at the white river by Augusta, Arkansas a couple times a year but I have been out of the loop for a good while now. I could care less actually. But one might stand to learn a thing or two if you decide to research it, or better yet, just go over there.
The last time I was out, someone has a 25hp Honda, but had made or somehow fabbed up a straight exahust that ran straight out of the head, outside of the cowling, just straight out. Loud and sounded odd, I do not recall it running much if any better than some of the other stuff out there; aside from just being loud. Still quieter than a lot of the 2 stroke motors though. Like I said, these guys are hard core, and mostly crazy.