Saw that review, as well, back when I was in the market for a good tele. The 1.7 TC has a pretty shabby reputation, though. Every review I saw said it's optically not as good as Nikon's other two, at least in their latest incarnations. That's why I only tested the 1.4 and 2.0.
You're probably right, that IQ will likely be pretty close between the different lens/TC options we're discussing here. Since that's what I am most interested in, I always test two options, both shot handheld, at maximum aperture and at a distance that's realistic for birding. One serves to figure out the lens' realistic minimum shutter speed, the other to test sharpness with faster shutter, i.e., in good light or at higher ISOs. Will report my findings when done.