RE: ASP 15MX Mono NEW BUILD
What it's called is port matching. It can be used on any 2- or-4-stroke engine for more efficiency. What that header flange is doing is choking the flow of the exhaust, the same thing as using too small of a carb on the intake.
I've seen other marine engines especially on RTR's that need it badly. If someone could give you free HP and RPM, this is an easy way to do it with just a few minutes worth of work.