Ok, so I got out for a second round at the range. I tried several flavors of slugs and found a common factor in accuracy performance, velocity. Slugs that push the high end, up around 1900 fps advertised, shot like..., well awful. 2-3 inches @ 50 yds. Stuff that runs 1500 fps or so is much better, 0.75 - 1.5 @ 50 yds. So as predicted, the Buckhammers were among the best. Others that were around an inch at 50 included the Federal Premiums and the Lightfields.
My USH has just around 100 rounds through it. I don't know if I'll see performance move one way or the other as the shot count climbs.