I am seeking experienced user feedback about the FOCUS performance and IQ comparison between the EF 300mm f/4 IS with a 1.4x ext. and the EF 100-400mm L straight up. I am going to buy one of these lenses to be used mostly for Bird/Wildlife and would like to get the best performer in AF and IQ at their perspective longest focal range (420 & 400). My #1 interest is IQ as I can work with whatever the AF has to offer. Much thanks!