What are you hunting, and where? Distances you are looking at shooting? I shoot Hornady 12ga 2 3/4" 300 grain SST slugs exclusively. My Rem 870 will shoot them inside 2" groups all day long, with a Hastings barrel and Leupold scope. I load 5, and is really accurate at all ranges. Shoot it in a couple of our short-range units here in Idaho. The 300 gr SST at 2000 fps is devastating. I have killed over a dozen whitetail and antelope, plus two cow elk with it. Have yet to recover any slugs.
If you are set on even more than that, find some of the old Winchester Partition Gold slugs. They were 385 grain partitions, at 1825 fps and really pack a wallop. They shoot good in my partners Benelli SBE, but only about 4" groups in mine.