Maybe I'm missing something, but I can't see a lot of benefits to the "compact shotgun" thing outside of a breaching tool. If I'm looking for compact but bigger than a pistol, it just seems stuff like the shorter SBRs/AR pistols or MP5s or similar have more benefits. This with the assumption that you want the shorty shotgun for indoor use.
I don't necessarily disagree with you here, but here's what I've found from recent experience:
The difference in maneuverability and handling in tight spaces or indoors between a 14" shotgun and an 18" shotgun is huge. I would say it's larger than the difference in handling between a Mk12 (18" AR) and an M4 (14.5") AR. It is pretty universally accepted that for work in close quarters or confined spaces an M4 is better than a Mk12.
12ga buck, up close, is a devastating load ballistically. It's probably closer to the fabled "One Shot Stop" than 5.56, although at bad breath distances, both are very capable loads for anti human work. There are overpenetration concerns with both rounds depending on your load, and dwelling construction. A 12ga is obviously lower capacity and higher recoil than a 5.56SBR or 9mm subgun, but the trade off is a significantly larger amount of lead going downrange per trigger pull.
So: keeping all that in mind, if you have decided that a 12ga is your HD/Vehicle Defense weapon of choice, all other things being equal, a 14" SBS will be significantly better in that role than even a 18" riot style gun. (this does not apply IMNSHO to PGO or Birds head gripped 12ga) It will maneuver better in confined spaces, and transition between targets better, and overall be better for the close in HD work. Like the SBR vs Rifle comparison it'll give up some range over it's longer barreled brother, but not a meaningful amount in the context of a gunfight inside a building or vehicle.
Now I still don't claim to be a shotgunner, and I personally still use an SBR (or Subgun) for my HD work, because that's what I train with the most, but after running my SBS, the .gov's SBS, a buddies SBS (the Genesis), my 18" 870, and a 1301 for several thousand shells over the last 6 months that's what I've found.
YMM, of course, V.
