I have a 5D3 and a 70-200mm 2.8 IS mk2
I use a 2xmkIII quite a lot and find the focus speed to be excellent when considering the loss of light.
I'm not saying it's blazing fast as it is without the TC, but it's not slow by any means (I shoot horse jumping etc without issue)
I would use it, especially considering the available light at an air show.
I would also use the expanded centre focus point, aircraft aren't that small to need tracking, and if they're that far away, don't bother shooting, save your battery until they're close enough.