I never shot factory Supers in my 400. But its previous owner might have. I always pulled the bullet and dropped the charge by a couple of grains. That was a pain in the butt. .38 Automatic ammo was too hard to find. Supers are loaded to 36,500 PSI. Don't know what Largo's are loaded to but 9X19s are currently loaded to 35,000 PSI, at least in this country. The +Ps are hotter still @ 38,500 PSI. These are SAAMI figures.
The interchangeability myth is just that, a myth.
The 400 was never designed to shoot any cartridge other than the Largo. The Steyr round will work because it has a slight taper. It will chamber and fire quite nicely. The opposite usually won't work. The straight tapered Largo round usually won't chamber in a Steyr pistol.
Trying to shoot 9X19s in a 400 will sometimes work, if the extractor slips over the rim during feeding. The 400 is a "push feed" system, the extractor snaps over the rim after it is fed into the chamber. If this happens, the extractor will hold the case against the breech face and the round will safely fire. But if it doesn't happen, the 9X19 round is pushed into the chamber and sits there ahead of the extractor, about .070 thousandths away from the breech face. The 400 has a free floating firing pin that is spring loaded, similar to a 1911. One of two things will happen next. 99% of the time nothing will happen. The firing pin will not quite hit the primer. But if the chamber is dirty and the 9X19 isn't pushed far enough into the chamber.... Guess what happens next....This didn't happen to my gun, it was simple metal fatigue, leading to a crack and failure.
Scout21, here's my shiner.