First let me state my big game credentials...I have none, so the following is just my opinion and heresay...
The Kevlar stock was developed for Dangerous African game for the benefit of durability in humid environments, light weight, and strength. So, it is ideal for the big boys.
The pros/cons of the rounds that you mentioned are...
1. .505 Gibbs-big proven stopper but expensive rifle and costly to shoot and big on recoil
2. .458Lott-based upon an earlier cartridge, the .458Watts, proven stopper at relatively low cost, but big on recoil
3. .404Jeff.-pretty good stopper, expensive rifles and costly to shoot, less punishing recoil
4. .416Rigby-good stopper, expensive rifles and pretty costly to shoot, less punishing recoil
5. Don't forget about .470NE, 9.3mm, .500A-Square, .458WM, .600NE, .700NE, .577TRex, .375H&H, and countless others that I have missed.
Are you considering one? My pick will be the .458Lott (probably the most popular) due to the less expensive rifle, ability to shoot .458WM/.458Watts cartridges and load with 45-70 bullets (not cartridges...just the bullet), and because it is a proven stopper. FWIW if you go with a CZ Safari in .458 caliber get the .458Lott not the .458WM because the cost is slight, the .458WM has a tarnished reputation (in Africa at least), and most importantly the ability to shoot both Lott and WM ammo. But it is a Lott of rifle.