Seems to me, taking on dangerous game is all about penetration, assuming a vital shot is made. Penetration is actually pretty easy to predict, its a function of sectional density (weight of projectile compared to its length) and impact velocity. (Note the word "impact".) Often overlooked, it is obviously a very important value on when boring through an elephant's skull or into a a big animal's chest.
Most bullet manufacturers publish the data for their bullets, but here's an online calculator: Beartooth Bullets > Ballistician's Corner > Sectional Density
Identically constructed projectiles of the same sectional density, when impacting an object at the same speed, will give the same penetration. Think spear vs. rock of the same weight, the spear simply retains velocity better since it will encounter less resistance due to its diameter. This is why the 500 grain .458 (sd. .341) pill will penetrate deeper than a .600 750 grainer (sd. .298). Perhaps the .600 will be adequate, but the .458 still goes deeper assuming both bullets stay intact.
Armed with this knowledge, we can compare rounds. Assuming one can impact a 900 gr .72" bullet at about the same velocity, with a sectional density of .248 it will not penetrate as deeply as a .458 Win or a .600 NE. The keyword is impact, for example according to Nosler their 500 gr Safari Solid is down to 1521 fps at 200 yards, about the velocity blkpowder50 was speaking about from the muzzle. Assuming a well-placed shot, is that adequate?
I'll leave that to more experienced hunters on the forum.
I will say, however, I spent the last week hunting with Safari Club's #1 World Handgunner in 2010/2011, Kris Johnson. I know he's taken buffalo with his .500 S&W revolver, pretty sure that doesn't have nearly the poop of your .72 smoke pole. #1 handgun lion, #1 bongo and a top 20 hippo with it as well (not sure about elephant).
I have to say, with backup, sounds like the makings of a great trip! And I'm pretty sure there's a PH out there that'll act as a true back-up, not a simultaneous fire, if you prove yourself and your rifle on some non-dangerous stuff (eland, etc.) first. They are there to get you out of trouble, not kill your animal for you unless they deem it necessary.