Although I can't prove it, I am guessing that this leverage has been responsable for most of Buffett's superior returns, not the fact that he picked great stocks. Would you dispute this claim?

Yes. Because Warren's most spectacular returns were long ago when he was the "cigar-butt investing guy", the brillant stock picker, something that works less and less as bigger you become, so that he simply at some point had no other choice than with help from Charlie to give that up and to become the "Buy great companies at reasonable prices" guy.

Confirmation that giving up his old and super-successful style was only caused by ever increasing size: Some years ago he said something to the tune of "If I had just 1 Million dollars I am convinced to (again) have 30%-50% yearly returns". That's not possible by picking great companies and holding them forever, but only by picking stocks and after getting some last puffs out of them which only you saw and nobody else, to sell them and pick the next ones.