I'm hardly an expert on leopard, as I have taken all of the Big Five EXCEPT leopard. Both of my unsuccesful leopard hunts were 21 days each, without a shot being fired at Mr. Spots. Will try again next year.
My leopard rifle is a .416 Rigby with an illuminated scope. The reason is that I mostly hunt Zim and there is DG everywhere. This year, we were walking back to the truck one night and encountered a family of elephant blocking our egress. On another night, a family of elephant literally walked up to the back of our blind before we became aware of them. On another hunt, two lions came very, very close to our blind and we walked back to our truck in the dark listening to them call. While I agree that a rifle in the .30/06 class is plenty for leopard, based on my experience, if you are in DG country, bring at least a .375 with a soft up and solids down in case you have the misfortune to encounter something large and unfriendly in the dark.
You should also consider your followup rifle, in case the leopard isn't dead under the tree. A .30/06 class rifle is insufficient for that task. I would think a 9.3x62 or a .375 would be a minimum. On my lion hunt, I followed up with my .470, loaded with softs.