I found a gel test online the bismuth penetrated 3” at 40 yards and the steel penetrated 2”.
That sounds about right. I have never tested anything ever with ballistic gel.
But I saw a video, once.
#4 bismuth at 1,350fps and #2 steel at around 1450 have near the same penetration. SWG user Randy Wakman has done testing with loads of all kinds, and I think he has access to s ballistics computer, (the decimal fractions give it away) so you might ask him for the real life, not quite guessing answers.
See that's the gimmick- you might think with bismuth you get less pellets per an ounce of same sized shot. And you do. But because of the improvement in penetration you can go down one size from steel and yet have more pellets with greater penetration than steel one size bigger.
Where it gets really crazy is with tungsten shot. You might use 7.8 gram per a cubic centimeter (g/cc) steel shot size BB, which runs out of enough oomph for canada geese at around 35-40 yards. With 18g/cc #7 (yes, #7) tungsten you can whack a canada as far as you can choke it, maybe 60 yards or more. But, it'll cost you a bunch of money to use it.