Full disclosure, I have never taken a big cat but plenty of my friends have and I have spoken with and read accounts of numerous Professional Hunters. The conventional wisdom is cats are very sensitive to high velocity calibers (and properly constructed bullets to match). There is also a big difference between a leopard and a lion. For me, speed kills baby so a 300 for leopard and a 375 for lion as fast as I can push them would be my recommendation.
I remember Ross Seyfried writing an introduction for the .378 Weatherby in one of the Nosler manuals about this very thing.
He said it left "chunks of lion clockwork..."
