Well there is a lot of confusion about Tele-extenders. I have been using them for years on everything from primes, mirror lenses even on my Hasselblad.

The thing that no one mentions is that if you add a 2 x teleconverter to you lens it cuts the resolution in half. But, if you compare two lenses (of about the same quality) lets say a 300mm and a 600mm. The 600 will have about half the resolution of the 300. The other thing to keep in mind is that it does not change the close focus distance of the combination, it allows you to do some close focusing work at a nice distance. It does this without the hassle of using an extension tube. And the thing about quality is that a teleconverter is a simple doubler.

So the manufacturer of the TC may not be as important as some would have you expect. And aftermarket TC may actually be more flexible i.e. the Canon units can only be used on some lenses because they extend deep into the lens.

And yes they do severely inhibit auto focusing on slower lenses But in my experience 600 f 8 or 1000 F16 is not someething that you are hand holding anyway.