Absolutely. According to my dof calculator, the DOF at 300mm f/5.6 at or close to minimum focus distance is about .22 inches (5.5 milimeters). That is actually a smaller DOF than my 135L f/2 wide open at the same distance (.39 inches or 9.79mm)
Thanks for the calculation, that's really interesting. I recently discovered the severity of the "small dof effect" when I was wondering why so many of my images are just slightly out of focus - and atm I think the reason is that focus & recompose simply doesn't work reliably anymore.
At the moment I have to take multiple shots and then select the sharp(est) one, but I'd be grateful for any hints for circumventing this problem w/o purchasing a 5d3...
Despite reports about the 6D being "center point only", I actually have a pretty close to perfect success rate using the outer points. I don't know that I would try it in really low light, but most of the time it works fine. I can't remember missing a lot with the 60D either, though. One your biggest advantages when you move to a 6D will be the ability to AFMA your lens to the body. On my two bodies, one is a -1W 2T and the other is 2W 2T. That little bit of adjustment can make a big difference.
If you are shooting narrow DOF shots off center for composition and have time, using Live View focus is a pretty sound choice. Slower, but highly accurate.