Putting a converter on reduces the widest aperture by 1 stop. As long as putting the converter on doesn’t make the widest aperture more than f/5.6 auto focus works.
The EF300mm f/2.8 with the converter becomes f/4.
The EF300mm f/4 with becomes f/5.6 (and still autofocuses).
Some lenses take converters better than others and the 2X converter is generally considered awful unless you have a really good lens to put it on and are shooting in decent light. Apparently the 300mm f/2.8 will take the 1.4 TC and the 2X TC stacked and still produce good images if you have decent light!
My problem at the moment is I am having trouble with the 7D, when the f/number is getting up above f/11 (ie lots of sun when shooting at 1/320 or less) diffraction is starting to do funny things to the image Thinking of chopping it in for something else at the moment.