I had a delimia where I could buy the mark IV for about the same price as the 5D III. I made the decision to try a 1D camera. As I had never had one before.
I am happy with the purchase. The 7D with the 300 II lens is just not a good match for me. Not that the 7D is not a good camera it is. But the AF is lacking and just makes the 300 a very poor performer. I tried everything I could think of. Including two trips to CPS in Thailand. Finally one of the tech's told me, just buy the mark IV. It really chapped me to have to buy another camera to see if the 300 would start to perform.
To make a long story short, it did the job. I used the 7D for so long I loved it and did not think anything could be better. The crop is amazing and the detail is not matched by the Mark IV. But the AF and keeper rate of the Mark IV made a huge difference in performance with the 300 mm lens.
Now I am very happy with the lens and using it with a 1.4 T.C. It has been a great investment. The more I use the Mark IV the more I like it.
So, if you have the 300 II and use it with the 7D. And want to increase your keeper rate with and without a T.C. I highly suggest you go to a ID series camera.
Go birding, have some fun. Great exercise and the easiest hobbies you will ever get involved with I think.