Upload & Sell: On
The 400mm DO is very portable and easy to put in your carry on bag. Easy to tote around during tours compared to the 500 f4. It is a great mammal and large birding lens. I have hiked 6 miles with it on my neck and a 22 pound toddler on my back. Couldn't do that with the other big whites. Definitely a 1.4 extender will be handy with brighter light. With great light, I drop in the circular polarizer, so make sure to rent this as well. With tripod sensing IS, you can take good photos of static animals at low shutter speeds, keeping your iso down.
With the 600mm II out, I would consider renting this over the 500mm.
The 300mm 2.8 and 400mm DO are about the same size/weight. So it depends what focal length you need. If you are doing wild life, most people will want the longest lens they can get, then add an extender. The 300mm 2.8 is supposed to be super sharp, but I wouldn't make that your main deciding point.
Crop body will give mildly better results IF you are focal length challenged, o/w full frame is mildly better. If you are used to 1D series, I would get the IV instead of the 7D. I don't have either, but I understand it has better noise, weather performance and you can AF at f8. The 400 mm DO can do well with a 2x, but you will need to stop it down a little to f10/11 - that requires a lot of light for anything that moves.
don't worry about the contrast and sharpness critiques of the 400mm DO - it takes great photos that are easy to post process. Some sample shots.