A few thoughts on the matter:
*I built a Contender SBR some years ago. Add a suppressor and it makes a nice, compact package. I especially like how easy it is to switch calibers, configurations, etc.
*I'm not much of a shotgun guy, but if I were to do a cut-down single-shot SBS to use only as a shotgun, I'd use a different host firearm. There are all sorts of inexpensive single-shot shotguns out there that would work great. You could build a couple of them, including tax stamps, for what it would cost to do one Encore SBS.
...also do I need a separate stamp if I decide to make it into a SBR as well?
*Will ATF let you make an AOW from a full-length, stocked shotgun? The answer is NO. This is because the definition of a short-barreled shotgun includes
"...any weapon made from a shotgun (whether by alteration, modification or otherwise) if such a weapon as modified has an overall length of less than twenty-six inches. " (18USC 921 (a)(6))
So if you cut down a shotgun and install a pistol grip, you have something that resembles an AOW, but that is still a "shotgun", since it was "made from a shotgun".
Based on this, if you register and cut down a shotgun into an SBS, and you later install a rifled barrel, the firearm is still a "shotgun", since it was "made from a shotgun".
(disclaimer: I am not a lawyer. This is speculation on my part, based on past ATF rulings, the text of legal definitions, and application of logic. That last part (logic) may or may not exist on the part of ATF. YMMV)