A 1.6 or 1.7 extender does interest me.
I photograph air shows using my 300 f2.8 IS II, much of the time with the 1.4xIII or 2xIII.
With the 1.4x the decrease in IQ and AF performance is nearly imperceptible.
With the 2x IQ and AF does take a hit and my keeper rate decreases, but I can still come away with some good shots.
If (and that's a big if) the 1.7x provided better IQ & AF than the 2x, then it may be a benefit for me. I'd have more FL over the 1.4x, yet still retain pretty good IQ & AF.
I agree completely on the AF part. F/8 AF is only nominal with some systems, and I think that the PDAF system is only seeing a small a mount of the incident light that it needs to see from oblique angles out of the exit pupil, demanding more contrast and/or light intensity, and operating too slow. Backing down to f/7 or so can greatly increase the needed light, without dropping to the magnification of a 1.4x.
There are many times when I use 1.4x instead of 2x because 2x causes too much hunting (birds in wildflower meadows and in trees with lots of leaves and branches behind them), and these are situations where I'd be cropping even with the 2x (or an imaginary 4x, sometimes), and the huge gap between my 1.4x and 2x kills me. That's where our opinions diverge, though; I don't believe in targeting pixel quality when using a TC. One of the main points of using a TC in my world is making sure that 100% pixel views are as soft as possible (by magnification; not TC-sourced aberration), so that I get a minimum of aliasing. I never pass on using the 2xIII because of "image quality" or "pixel quality". Loss of AF or too narrow a FOV (and possibly too dark of an OVF in very low light) are the only reasons not to use it.