The only way I managed AF with a 50-500 (of which I have had 2), was by using a cheap 7 pin kenko TC or taping pins on a 10 pin TC. It wasnt very reliable except in good light with high contrast subjects using the centre focus point, even on the 1DsII.
However, doing this just about gave you AF using a 2*TC.
The TC use I referred to in my previous post was with a sigma 500mm f4.5 prime (Sorry if that wasnt clear)
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