Okay, I've seen all the data on SBRs.  I haven't seen anything out there about shotguns.

Anyone have any info on the compromise in ballistics on say a 12 gauge with a 14" barrel with buckshot?

This is an old post from subguns that I saved, I don't remember the original poster, but here is some data.  

He apparently took a hacksaw and a chronograph and fired, cut, fired, cut etc. and recorded the results with Aguila 12 Mini Slugs, Winchester 3" Mag. 1 oz. slugs, and Rem. 2-3/4" "Slugger" 1 oz. slugs. Barrel lengths were 31", 25", 19", 13", and 7".
At 25":
Aguila/Win./Rem.
1137 fps/1685 fps/1498 fps
At 19":
Aguila/Win./Rem.
826/1542/1411
At 13":
Aguila/Win./Rem.
1139/1433/1324
At 7":
Aguila/Win./Rem.
1024/1134/1131

What the heck went wrong with the Aguila at the 19" mark?

Don't know, what I remember most from that thread was that after 10-12" you did'nt gain much in velocity.

This makes me want to get an SBS to match my future SBR

Google Vang Comp.  I hear their work is priceless.  I already know I am having them shorten and crown mine for $55.  I may have them do the back bore for the lightened recoil and tighter patttern.  Might.......hell, in the back of my mind I am sure I am having the Vang Comp system done to mine.  

I am planning on a 13".