There is another fun formula on the Beartooth Bullets site.
http://www.beartoothbullets.com/rescources/index.htm
Called "Thornily Relative Stopping Power" .
It places the 16 bore at about the 375 H&H level at 1600 fps MV, 1400 striking velocity.
This agrees with Forsythe stating the the 16 bore is about as small as anyone would use on dangerous game.
My flintlock 16 makes 1600 with 140 gr (just over 5 drams) of FFG Swiss and it really is a power house.
But I think hardened lead balls would be needed to produce best results on game larger than deer or elk. The 458 is actually in the realm of the 10-8-4 bore in actual use.

Dan