Im not a MC owner (CC fanatic) and an EE by degree... though I do deal with mechanical stuff at work a fair amount. My recommendations on the sealant are based on practical experience (restored/rebuilt several boats from the hull up). Theyre consistent with the advice I have followed from people much smarter than I am. Silicone just doesnt belong below the waterline.
I use 5200 for pretty much everything now, but Boat Life Caulk works well too. Never used 4200 but Im sure its good stuff based on the other 3m products Ive tried. Ive had to remove running gear that had been installed with 5200 just months before and its not *that bad*. I cant think of any reason you'd need to remove an exhaust port... if you use the proper sealant, you shouldnt have to re-do it in your lifetime.