I know there are a few here who shoot airshows with a 500. Might want to review some airshow shoots in the transportation forum.
I very rarely use my 300 with a 2x TC. Wide open its a bit soft, AF gets very slow, and IS becomes less effective. It works, but I feel the need to shoot at f/8 and that gets a bit slow for things like an air-show. A 1.4x is fantastic with this lens, a bit sharper and a stop faster than the 100-400.
edit: by the way. Any of your choices(except maybe the 300 + 2x) will produce fantastic results. The 1-4 is probably the cheapest rent, so I'd strongly consider that lens. Then, I'd jump all the way to the 500 for best images.
edit 2: Just looked at your gear, and noticed you're shooting with a 5d. If it doesn't break the bank, definitely go for the 500mm!!! And maybe bring a 1.4x TC.
edit 3: Hey what's one more
Haven't been to an airshow since last year, so the topic got me looking at my shots. Here's a couple formation shots at 400mm, on a 1d, to show you can't be too long at an airshow.
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