I put a set on my 28 Nordic Heat to take the guesswork out of new noise laws on Lake Havasu.
I had the stock tips welded to the mufflers.
Installation was easy.
496 Mag HO.
Much quieter, I can somewhat carry on a conversation with front seat passenger while cruising 50 MPH.
May have lost about 100 RPM, but I don't perceive any loss of time to plane, accelleration in mid, etc..
Some others I know who have installed Zoomers have reported they actually saw performance improvements...but you'll want to consider that subjective information.