David, I think that you did great. Trying to MF at 800mm is really tough with today's viewfinders. They're just not made for MF. There is some luck involved. Before my 5D MkIII would AF at f/8, I tried MF a couple of times at 1000mm. I thought the subjects were sharp, but they hardly ever held up to close scrutiny.
BTW, try a 1D X. It'll blow you away with its AF. It locks in instantly with TCs attached. My 5D MkIII will lock in, but after a second or so. The 1D X is like .1-sec.