1 -- Thousand Oaks 77mm threaded filter on a 5D3 cropped from 400mm 5.6 non-IS, at ISO 100, f/5.6, 1/640:
2 -- Cropped from 400mm with 1.4x TC, ISO 100, f/8.0, 1/320
The above are 1:1 image crops. I'm not able to discern an IQ difference when I look closely. Can you? I'm thinking of ditching the 1.4x TC to allow for nine focus points vs. one if I use the TC. I'll take focus with auto, and then switch to full manual focus using gaffing tape to lock my lens at infinity for subsequent shots. The 5D3 at f/8 allows only the one center cross-type focus point, unfortunately, which can get tricky when it is that dim.
Another consideration is how wide the corona might become during totality. With the TC, I might miss some dimmer tendrils at the boundary of the frame, as they can sometimes be seen eight solar diameters out (when bracketing).