Well I was pleasantly surprised when I slapped a Tamron AF 2x TC on my 6d with the kit 24-105mm F4L. (This TC reports).
Outdoors and in Bright light, the autofocus at f8 works just as normal. In average light, it is a bit slower to lock on, but not by a whole lot. In dark situations where the 24-105L alone would lock on, it is hit and miss with the 2x TC.
I'm waiting for a 300mm F4L to arrive to replace my 70-300IS since 99% of the time I'm always at the long end. If it works just as well, I might have a good long reach option for my birdwatching without spending thousands.
I read a rumor about f8 being supported for the 6D in a June firmware update. What I want to know is what exactly would this firmware update do given that it already kinda works with this f4 lens. Would it improve even more ?