Congrats on your purchase. I bought the 2X Mark ll Extender to use with my Canon 70-200 F4 IS USM. Even though the lens is now a F8 aperture lens I get good results with my 7D. Better than when I first bought it and used it on my 40D.
The lighting has to be optimal to get good results. Brightness and angle to the sun become more important than shooting without it. I can hand hold the lens and extender and get a good usable pic using manual focus at F8. Practice,Practice,Practice. I kept trying on occasion and got my best pics this past weekend shooting Eagles in the same tree as other days. but from the opposite side. Standing North and aiming south not South aiming North as on previous attempts. In camera the pics are sharper on the 7D than on the 40D so camera does play into the equation. My next step is to use this combo mounted on my tripod with a cable release. Mounted on your 5D lll you should have no probs.