First some legal requirements, e.g. Namibia:
The fundamental purpose of the Namibia Professional Hunting Association (NAPHA) is to enhance and maintain, by effective management, an organizational infrastructure that can serve professional hunting members, clients and other interest groups.
www.napha-namibia.com
As per the Nature Conservation Ordinance No 4 of 1975:
No person shall use a revolver, pistol or automatic firearm when hunting game or use a firearm of which the bullet has an energy at the muzzle of the barrel which is lower than the following when hunting a species of game indicated there under:
Small Game:
(e.g. Dik-Dik; Steenbok; Duiker; Springbok)
Minimum caliber: .243 (or equivalent caliber in mm)
Energy: 1350 Joule
Medium Game:
(e.g. Hartebeest; Oryx; Wildebeest; Kudu; Eland and all exotic species)
Minimum caliber: .270 (or equivalent caliber in mm
Energy: 2700 Joule
Dangerous Game:
(e.g. Elephant; Hippo; Rhino; Buffalo; Lion)
Minimum caliber: .375 (or equivalent caliber in mm)
Energy: 5400 Joule
No solids are allowed as per the Nature Conservation Ordinance No 4 of 1975 on any other species than pachyderms (Elephant, Hippo, Rhino).
Hunters visiting Namibia may import 60 cartridges per caliber.
-------------
This means in Namibia anything less than .270"/7mm is not legal on medium game (including Leopard).
And the legal requirements are only the absolute minimum, an experienced PH would recommend other calibers for a trophy hunter with limited days for the hunt.
In other African countries the Leopard is listed as dangerous game with a legal minimum of .375" or 9,3mm or >5200J etc...
If you´re hunting Leopard, a 7x57 will kill it IF (!) the first shot is placed correctly.
If not, searching a wounded Leopard will normally end in an attack of the cat, which is no fun at all.
So the next thing you have to consider: how precise is your rife?
And: which scope do you use?
As the hunt often takes place when it´s already dark you need to rely on the best optics you can buy for money. Swarovski or high-end Zeiss - if you have a scope assembled in China with poor glass...
If you want to hunt a Leopard you either know you receive the best rifle/scope from your PH (who considers also hunting laws) or you bring your own rifle with you.