I chitchatted with Leland at there booth at the BHA rende. When I questioned him on the performance, especially at distance/lower velocity he walked me through a paper from Germany that looked that concluded that although the call copper doesn't expand as much at low speeds they typically retain either a flat or convex nose which still creates as large or larger wound channel do to the increase energy wave propagation as the larger diameter lead bullets since almost all lead bullets end up with at least a very slightly rounded nose.
I was holding back on switching because I couldn't get any to choose very well, thankfully I threw it out to the HT community on several people gave me there recipes, which are significant improvements over they groups I had gotten on my own.
The reason lead bullets kill quicker is the shrapnel effect of bits of lead flying everywhere. Its no secret that the fastest killing lead based bullets are bullets like the Berger VLD which shed most of their weight.