First, I am new to this forum but it appears like there are some interesting topics.
My opinion:
A large male Brown Bear could kill a Lion or Tiger. I'd put money on it. Also, if a 45/70, properly loaded, could handle a large bear, why not a Lion? I would think it could certainly handle a Leopard. Hell, Leopards aren't that much larger than our North American Mountain Lions, which can be dispatched of with a 357 Magnum or 30/30.
I understand that many PH's may not want their clients to use less than a 375 HH on the big 5 (and I could certainly understand when it comes to Elephants, Rhino's and Cape Buf - actually 375 HH is probably marginal).
Incidently, I did see the thread on the 45/70 vs 416 and I mostly agree with the PH's point of view. I also understand their are minimal energy requirements for hunting dangerous game in some African countries.
However, when it comes to the big cats, I don't see why a hot loaded 45/70 or 450 Marlin could'nt be used to dispatch these cats at closer ranges. In my mind, 5 rounds that could be cycled quickly by the lever action would be nice as well, wouldn't it?
If these calibers wouldn't be accepted - how about more powerful leverguns like the custom jobs Wild West Guns makes:
457 Magnum, which also will chamber the 45/70 &
50 Alaskan.
Check out the link below (power chart)
http://www.wildwestguns.com/Ammunition/PowerChart.gif