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I've been at Barksdale AFB for 10 years now, and we've had an airshow every year. Of those 10, I've shot about 5 or so. The 100-400 is just fine.
If it's too short, that means the plane / jet is too damn far away anyway, so wait until it's closer for more interest
Also, strongly consider an ND filter if you're expecting lots of prop planes. Getting good prop blur is almost impossible without one (you'll be at something like f/22 and ISO 50, not the best settings to have).
A 3-stop should be sufficient.
If I can shoot an airshow with a crusty Pentax K20D and Tamron 70-300 lens, surely you can pull it off with your proposed combo
This one was at ISO 100, f/18, 1/100s. A77 with 70-300G. Free crunchification courtesy Flickr downscaling.
210mm or about 315mm in FF terms.
B17 Fortress by J L Smith, on Flickr
If it's a decent air show and you're not 1/2 mile away as the crowd, almost all the good shots are the "showman" shots where the plane angles towards the crowd and is sufficiently close enough that an incredibly long tele isn't really needed.