Perhaps it's a camera setting.
I have to say, the rendering of flat areas of colour is falling short of expectations.
Should I reset everything and let the camera sort out the technical stuff?
Anyone got any suggestions as to settings for this type of photography as i am back at the mach loop in a few weeks?
Auto noise reduction, yes/no?
My opinion/assessment: Your 1.4x TC is not playing nicely at all with the 100-400 II lens you have/had. The plane should be MUCH sharper than the samples you've shared here. So micro adjustments would be highly recommended next time you head out to the loop.
For aviation photography, I will shoot either in M or Tv mode, but setting ISO to auto and let it rack the range. I wouldn't bother with 1/8000s. I find 1/2000s generally good enough to freeze jets, unless they decide to zoom by near sound barrier and that I'm having a terrible day at panning. But with 100-400 + 1.4x, you likely won't get much of a panning opportunity since I would imagine you just getting half the plane, if not only the cockpit as they make the bank.
IS set to mode 2.
Exposure comp I'd usually dial in +1/3, but if plane is up high against the bright sky, I'll go as high as +1.0 EC.
Auto noise reduction is a JPG thing so if that's all you shoot, then yes. Otherwise, I deal with my RAW files in LR, but I rarely dial in noise reduction until ISO 6400 or higher.