I think the advantage of SS is that it shouldn't corrode/rust . The build up in exhaust ells is a combination of rust & lime scale.We have found that you can only acid wash the elbow once. The build up is calcium, in some form (but soluble in acid), and carbon (which is why its black). Unfortunately rust or iron oxide also dissolves in acid and as the elbows corrode or rust the simple technique of removing the clogging also removes some of the oxide, leaving a fresh surface to rust. I think the steel is also soluble, slowly, in acid so the act of cleaning need to be done with caution - it might not work (or it works to well) and you end up with a hole in elbow, been there, done that!
Stainless seems the way to go, especially if you intend keeping the yacht. I have heard tell that stainless does not result in the build-up or clogging in the first place - but I actually do not know of anyone with one to verify this.
You may still get some mineral scale build up but no amount of rust flakes mixed with it,meaning a lot longer time before significant blockage occurs. This is personal opinion only-it will be awhile before I have actual results.