100-400L IS verses 70-200f/2.8L IS with 2x extender

Alot will depend on how deep you peep. If the frame is filled with the 2X on the 70-200,and the overall scene is evaluated-not just one tiny target,it does a very good job.
AF will slow down,but an easy way to help this is to set the AF distance switch to what you're working with-it does help cut down on the "hunting" considerably.

my 100-400L IS AF does not work with the EF 2X II (but my 70-200 f/2.8 IS does)

Both works with my tamron 1.4X

Is the AF not support? I see B&H list "impossible"
http://www.bhphotovide​o.com …arts/canon2xExt​ender.html

but still like ppl are talking about its working... am I missing something here? TIA

BTW, I have 40D body.

^^^ i think it involved taping some of the pins to get it to AF... not sure.