I assume you're refering to 100-400mm f/4.5-5.6 L
With any canon compactible 2X convertor, 100-400mm f/4.5-5.6 will become 200-800mm f/9-f11.
I believe all canon bodies will loss auto focus after f/5.6 except the class belonging to Eos 3, Eos 1 as they still AF at f/8.
In very bright condition, AF is still possible at f/11, however the AF cannot track fast moving object properly.
An 2X AF will be useful for a f/2.8 lense since the max aperture is f/5.6 after adding the convertor. All canon bodies can AF with f/5.6 if the 2X AF is made for EF mount.