Seriously though, his answer does hit the point rather succintly...you need the longest lens you can afford. Of course not the 1200mm lar. Even if you can afford one you will break your back just carrying it around.
To fill the frame, lets say if you are shooting birds, ideally you need at least 500mm or more. The Sigma 50-500 looks like a decent choice. 100-400 L ok lar....but still a bit too short for birds. Else, if u got the moolah, maybe go for the behemoth Sigma 300-800mm instead?
Well perhaps to compromise get a 400mm plus a 2xTC lor.... sacrifice some f-stops and DOF and bokeh lor... that's my thinking at the moment.... a Kenko might be good enough? Unless u like Canon's own TC lor...
What kinda of 2x teleconvertor are you using ? Some like vitacon stated 2x but it is actually not even larger than 1.7x..
Besides.. if using a 500mm lens on a non-full frame DSLR.. it will be 800mm :devil: