Also just reading this now and thanks. I'm in that transition zone at f/6.7 all the time as my 500 f/4 VR and TC17E is glued on to my D800E pretty much permanently. It's a sharp combo as you see on this Olive-sided Flycatcher and Solitary Sandpiper. I love the 1.7 for that little bit of edge over the 1.4 on reach and yet still razor sharp. I am not convinced the 2.0 would be this sharp and I wonder if f/8 AF would be responsive enough for my needs. I usually only have seconds to get these shots so responsiveness is part of the total package. My experience the the TC17 is perhaps slightly contrary to reputation, but I have had great success with the TC17 over the years on at least 4 different cameras now (D300, D7000, D4, and now D800E).
I use AF exclusively but it does explain why I have to be careful sometimes that the rig does not just completly miss and unable to focus at all (at which point I MF it close enough to the ball park and then re-press the shutter to get the proper AF).
Likewise I don't like the 1.7 combo on a flying bird, prefer the TC14 then, which would be explained by the above