You already enjoy a crop factor with the DX body, so that 70-200 is already acting like a 100-300 without the TC. I'd still use the TC because airshows are like many vehicle races - "there's no replacement for displacement."
Given the MP count, you can use cropping in post-process to compensate for framing during the airshow.
Focus is key because of the limited amount of time you have to find the a/c from the middle of the crowd using your eyeball, switch to the VF and coordinate focusing, timing and framing to get the shot you want. Since you have the MP to crop, pick focusing over proper framing (i.e., 400mm long end) since you can compensate for that. You can't compensate for slower focus.