I thought Leica's 800mm was expensive at 18K or so (price is 3 years old) if it doesn't have IS the only advantage is the AF. You would be better of buying a group of refractive telescopes at 4K a pop and using them. BTW they are ALL hand made, which doesn't add a lot to the cost (your are paying for the design time, Mechanical Engineering time, etc and amortizing it over a few production units).
I was out shooting alligators with my 600 f/4 this weekend, and stacked the 2.0x with the 1.4x on my 1DM2, which resulted in an effective focal length of 2,184 mm.
Results looked good in reduced sizes, but at full 100% size, I could see I had to improve my shooting technique (and it was on a Gitzo 1548 with a Wimberley head). Mirror lockup and a cable release will be used in the future for this combo.
Bottom line, the Canon teleconverters were designed to work best with the long prime Canon teles, and accordingly, you can stack them (in good light). Try it, you'll like it!
Here is a sample of one of the 2,184 mm shots of an alligator:
