It's almost impossible then. Does anyone use a Canon 300mm with a 2x converter? I have trouble focusing. The only way to AF is to select either of the extreme focus points manually and pre focus on a point. It is impossible to us AF when pressing the trigger. The lens will just go hunting.
it's normal, I was using an EF 75-300 IS sometimes with the Canon 2x but it's like you said, lots of trouble, out of 10 photos maybe one satisfactory and never razor sharp images, always a compromisse between long range and average quality.
I use a 300mm f4 with 2x (and sometimes 2x & 1.4x convertors) and just use manual focus - at long focual lengths, I don't find manual focus too much of a problem as the difference between in focus and out of focus is readily apparent. Try using MF for a while - a little practice and you'll be fine.