I don't know about steel cased ammo but my buddy's Mini-14 beats the crap out of brass cases.
As far as accuracy goes, most commercial 55gr FMJs are about a 4 MOA round from most AR15s. I have owned 4 AR15 rifles (1 Colt, 3 RRA) all would shoot a 4" group @ 100 yards with Winchester, Federal, Privi, and S&B.
Each of those rifles had a favorite match ammo that would bring groups around 1". The Colt would put 5 rounds touching with Federal Premium 45gr. But I can't find that ammo anymore and I don't have that upper anymore.
If you are going to shoot steel cased ammo I would not expect accuracy results worth speaking of, regardless of rifle.
I only have experience with one Mini-14 and I have never been able to get better than 6" groups with it @ 100 yards. Bear in mind that the rifle wears a scope that I cost 49$ and was fed amateur reloads (there is room for improvement).
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.