However... other than the obvious 3rd party "issues" most have... speaking for the zoom length involved how would you of rated a Tamron/Sigma 150-600 to have performed. Did you ever feel 600mm would of worked out better?
Talley, I've never shot the Sigma or the Tamron; however, I have edited wildlife shots from the Tamron as shot by my friend on his 6D. I was pretty impressed. As I mentioned earlier, I had a 7D MkII body so if I needed more reach I used the 100-400 on it to give me distance. The only time this was really needed was for sunsets to make the sun really big through compression. I had a T/C but never used it.
Remember, my trips were all on private reserves where off road tracking was possible, so we often shot at 300mm or less. If I were confined to roads as in Kruger or Chobe, the extra reach of a 600mm would have been useful. I have a 500 f/4 and, as noted, a 1.4x T/C, but for the last few years, that's been too much lens on these private reserves. I do understand that serious birders will like that extra reach. I shoot birds, but am not really a birder of note. I just like colorful birds.