Rent me a 300 f/4, and I'll be better able to answer that question. I have to tell you that I would be perfectly happy with some of the 1st photos you posted, including the hawk eating the skunk (except as usual, I would have preferred a slightly different crop).
I'm sure that the 300mm f4 is a sharper lens than almost all zooms. You'll need to check whether any particular TC is compatible with any particular lens. TCs will affect exposure by a stop or two, and the DOF. So you have to see what your total needs are before making expensive mistakes like I have done.
Everybody has different opinions. I presented a photo for a live critique before a bunch of high-end photogs (It's of a duck that I think is in my gallery). I presented it to see if they thought I should spot remove a floating feather. The major criticizer went into a tirade like I had done something terrible to him. He said that if not for one thing in the photo, it would be totally worthless. No one in the audience had the guts to say a word. Guess I'll have to give back all that money I've made on that image.
So my advice is to get the best glass that you can afford for the purpose for which you want it. Then study techniques from the best (in person/on-line/or DVD), and practice, practice. Then share on here to learn.