I played with the 1.4x TC on my 100-400L on the 5DIII in February before the 5DIII could AF. My conclusion was that photos cropped in post had as much or more detail. I was not impressed with this combination and sent the 1.4 TC back. I've been using the 5DIII and the 100-400L together however, and they are a great pair.
BTW....I am another one looking at the 300 f/2.8....almost pulled the trigger last night...
I'm keeping the 1.4x TC. for now, I'll probably be using it with the 70-200 II and, in a pinch, with the 135L.
Anyhow, the TCs will be essential once I get the 300f/2.8 II ... So no point returning it, I might get the 2x in the meantime
I have 878 images taken with the 100/400 + 1.4TC, so it gets some use but its not my top lens. Its quite capable but wont take the best images.
Now turning to the 300 f2.8 It works fine with the TC both 1.4 and 2. These will take the best images and are my top lenses.