Am I doing the right thing?

The 400mm F5.6 is very sharp and very fast to focus. I use it for wildlife, particularly birds for which it is ideal. I don't go to air shows so it may be be different but for birds in flight I never find 400mm is too long. In fact for non-flying birds I will often add a 1.4X multiplier and hand focus, the 640mm option can be quite useful.