Even with a 12.5" bbl you've already lost some velocity, but that isn't actually the worst part. I've been playing a bit more with my 12.5" shorty (and no, not for sale... love the thing in spite of its limitations) compared to an 18.5" cylinder bore, and the spread is almost double at 10 yards with bird or buckshot.
The stopping power of shot against anything larger than a game bird comes from the amount of shot that impacts within a relatively small space. Start spreading out that pattern, and even if the shot is travelling at the same velocity, it will have less effect because less shot will impact the target, and even if it does all connect, the force will be distributed over a wider area, greatly reducing the effect.
Still a dang cool shotgun, though. And as for the $1K asking price - probably fair, for the quality of gun and parts you're getting. Comparing the Blue Line or DLask shorties to a DA grizzly is... No comparison. A DA Grizzly is a Ford. It'll do the job reliably. A DLask or a Blue Line is a Porche - it'll do the job reliably, quickly, and with style.