I do use the "old" 1.4x TC with my 100-400 and don't get "terrible" results, although I use it with my 1D3 and so get decent AF (center point only). I'd compare the results to what I got when I used the 2x TC with the 300 f/2.8 IS...
Here's an example:
The dang thing was still too far away for decent framing, but at least I could get a small print from a close crop:
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Another one at a similar distance:
Of course, I'd rather be shooting with a 600mm and use the TC so I could get a close enough frame to be able to print these large, but you make do with what you have