Kent also makes a T-M shell with 1.25 load, 1400 fps, which produces 34 ft/lb (7.5 lb gun, default values wad and powder). This is only 10% higher than c. 1922 Super X at 1.25 & 1330.
I also talked to an engineer at Hevi-shot, who said that the Hevi-shot Duck were only recommended for steel-ready barrels (the shot are a mix of tungsten, nickel, and iron).