Okay, so I just got this lens, and I'm jazzed, okay, I realize that. But, I put it on my 6D and took some pics of a building not quite 1/2 mile away at 300 mm and 600 mm with the 2X TC III. There are lenses that don't seem to match up too well with the 2X III (such as the 70-200 II zoom). But, that would NOT seem to be the case with the 300 mm 2.8 + the TC. WOW. You can see the screws in the security lights from nearly a half mile away. Okay, I'm impressed. (And these were just shot out of the box on a tripod, no remote exposure, totally auto mode shot...). (Click on the big thumbnail on the right to see the full resolution).
http://optimagroup.smugmug.com/Canon-ef-300-mm-f28-USM-IS-plu/
http://optimagroup.smugmug.com/Canon-ef-300-mm-f28-USM-IS-plu/