For reloaders, the 3.85" mag of the CZ 550 Magnum and the long throat of the .458 Win chamber allows loading .458 bullets such as the Barnes 450gr and 500gr TSX and the new Woodleigh Hydrostatics to a COL of greater than 3.6" giving the same powder capacity as the .458 Lott(with the same bullets).
For non reloaders the .458 Lott chamber is designed to also fire .458 Win ammo and only a simple reaming of the Win. chamber is required.
If the Lott does not kick hard enough. The action can be rechambered to .450 Rigby,.450 Dakota or .460 Weatherby. The rails ,bolt face and extractor need to be modified.
If that is not enough,in the .458 bore you can go up to the .458-577 Wells but there are extensive modifications required such as in line feeding.This is the largest capacity case for this caliber in a CZ 550 /BRNO 602 action. 500grs at 3000fps or 600gr at 2800fps. Not only will this chambering give you bragging rights but it will allow you to save on ammo by killing, if they are properly aligned, 3 elephants with a single shot.
Then there is the 50BMG upgrade. Have a custom 12 lb sporting rifle built and load some 600gr bullets backed up by 240grs of powder for 3100fps. You might want a muzzle brake on that one.