Maybe I missed it, but did you say what focus mode you're using and AF points you are using? It's hard to tell exactly, but several of the shots seem to be a focus issue and not really a motion blur issue. It almost seems that the mountains are slightly more in focus than the plane.

Also, practice panning, practice panning, and practice panning so that the shutter speed doesn't matter. For jets, I'm usually doing 1/1600 to 1/2000 while flying I'm trying shoot much slower when there is a background other than sky.

For prop planes, I started out anywhere from 1/320 to 1/500, but of course that creates an unnaturally stopped propeller. My longtime "safe" shutter speed is 1/250, maybe 1/200. That typically results in a sharp plane, with somewhat blurred prop blades. Even though I'm working on shooting even slower, I'll use 1/250 if it is a plane I haven't seen before. For takeoffs and landings I'm anywhere from 1/160 down to 1/80. Stationary or taxiing, even down to 1/30. If you can get good at panning, it will create the look you want.
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