Interesting, which lenses do you use it with Kev? The review linked above give the F4 primes a very low score with this TC.
I need to make my mind up first mate, it is a lot of money for a TC. I have the 1.4 and the 1.7 and both are fine on my 300/500. I know it is only one review, but he ain`t impressed, be good to hear if anyone has the 500 and uses it with this TC.
I have
Bought one to test after all the rave reviews, tested on the following lens.
70-200vr11
300 vr1
500vr
my findings tested at thruxton and with some wildlife.
the 70-200 worked well, auto focus slowed a little but nothing to be too worried about, contrast and sharpness and colour rendition suffered slightly but with with bigger targets ie motor sport, air shows etc unless you are a pixel peeper it works rather well. Anything a distance with wildlife however showed a distinct drop off in detail much more than my 200-400 VR
300mm VR it worked really well, little loss in detail but again with bigger targets it was acceptable, sharpness was very good considering it became a 600 but did benefit from shooting at F8 autofocus was marginally slower but would lock on things like birds in flight so very usable.
500mm my opinion don't bother, big loss in IQ especially in the fine detail, auto focus slowed to a crawl, viewfinder very dark and keeper rate fell through the floor, would not acquire birds in flight. The other issue was you need to shoot at F11 to pull back some of the quality, couple that with increase shutter speeds because you are at 1000mm full frame or 1400mm crop rendered it pretty much useless hand held and needed to locked down really firm in order to drop shutter speed without inducing camera shake, my point being it is not often you can shoot F11 and 1/1000th sec plus in our lovely UK weather

I also suspect the same issues with the 300 F4 but have not tried one so cannot really say.
For me I sold it within days, nice to test but it was not an addition worth keeping, however everyone has differing ideas on what is and is not acceptable. For those wanting a 140-400mm zoom or a 600mm on the cheap and I use that term loosely

then yes it may well be worth a punt. i do however feel it's and improvement on the 1.7x which i do not rate at all. Hope this helps and again this is my opinion and may differ from others.