the 800 f5.6 + 2x TC give you F11 (2 stop higher).. at F11, you'll lose AF
I see no real need for the continued existence of the current 800/5.6.
The 800 with the 2x TC gets you to 1600mm, can you stack TCs on the 600? I don't own a 600, 800, or a TC, so please enlighten. I think stacked TCs is not a desired option.
Exactly. Also, the current 800/5.6 + 2xIII combo is optically pretty weak. But a new 800/5.6 that delivers optical performance with a 1.4xIII that's equivalent to the other MkII's with the 1.4x, that would beat the 600 II + 2x and likely have a market.
Stacking TC's isn't possible (physically) with the MkIII versions, although a 2xII can be stacked with any 1.4x or 2x behind it, AFAIK. But the optical results are usually not good.