What is the latest consensus for the best "third-party" TCs these days? Have been using a Tamron 1.4x SP AF TC for many years but it probably could use upgrading. (Yes, am aware of light loss, AF troubles, and those things, but that's OK.) Ah, but ... if only Canon made such a thing, I'd be all over it. Primarily shooting with a 6D which I love. Suggestions?