I have used the Brenneke slugs on perhaps 50 deer and they were my favorite as far as terminal performance (as in fist sized entrance and exit). The 3in versions (both std and the gold "rifled barrel" slugs) are among the most powerful (recoil wise) in twelve gauge. I mostly used the 2 3/4 1 1/8 oz., and a few 20 ga 2 3/4.
Accuracy wise, they are pie plate performers from 50-100yds, depending on the gun. Most fosters have always shot better, but do not penetrate near as well. Call 75yds the limit to get exits on shoulder shots, past that the slugs usually stay in.
The problem with slugs is that they are barely stable as range increases and penetration suffers (accuracy is bad too), much like a wadcutter handgun bullet. Up close, slugs, in particular the Brenneke, is very potent.
Having never fired one at DG, you'll have to ask someone else!
