I'm the guy with the mentioned Jeffery .450/400 3" hammergun.
It is in very sound shape, although not quite new, it is on face, tight and well cared for.
As to speed of shooting. It is not particularly slow, as it has a top-lever and rebounding hammers. It can also be cocked absolutely silent, so when stalking the game will probably be unaware of presence, and you can through a speedy reload cock both of the hammers with the hand, if you practice..!
It is really not more unsafe than a cocked rifle of modern make i e Krieghof or Blaser S2, nor is it slower in my view.
Just because you see the Hammers doesnt make it a dangerous gun, as opposed to a hanmerless. Which way you point and handle the gun determines the danger... I know hunters I wouldn't be close to unless thay had a wooden stick, let alone a firearm...
In all shooting the first shot counts. The second is insurance if the shit hits the fan, but must be used wisely - i e short short range ob invoice game to make sure you hit, faster on outbound game. The situastion needing reloading for the 3-4 shot with a double is more imaginary? You have really screwed up if you need to pump out heaps of shots....? Especially today with linited bags and PH to back up. The hunting I have done, not much dangerous game but a bear, has taught me to use the first shot wisely. Saves a lot of trouble.. As long as you know your rifle well, and operating it is second nature, I see no problem hunting dangerous game with a hammergun. It comes down to practice.