How are you going to attach it? Weld on a fitting?
Yes of course it will increase back pressure, even if you donít put it in the water
it will. This will cause it to run hotter at least.
Ever notice your outboard
has two exhausts? One above water
that is used at low RPM
and the one thru the prop hub.
Maybe that is to keep from pulling water up to the engine
when the exhaust cools I donít know.