I had the 100-400 and go to a lot of air shows. It's a great focal range for that. But yes it's not as sharp as many newer lenses. I was always stopping down mine to f/8 without the 1.4 TC or f/11 with the 1.4 TC. Stopping down one stop made a drastic difference in sharpness with my copy. It was about a coin flip whether the TC was better than just cropping. And this was all on a 24 MP sensor.
One thing that really bugged me was the mechanics of the external zoom. Mine would suffer the creep when holding it face down, even if the ring was on the "tight" setting. And my arm would get tired from zooming in & out many times over the course of a day.
Otherwise I got lots of great shots with the 100-400 at air shows. It is a really good lens.
The 70-200 GM II was a major improvement in size, weight, performance, and image quality (except not at 400 mm). I can't see any IQ loss with the 1.4x TC, and the with the 2x TC it's on par with the 100-400, except the 70-200 with 2x TC has more color fringing.
So if Sony could come up with a 100-400 GM II with the modern optical formula similar to the improvements in the 70-200 GM II, and make it an internal zoom, and keep the weight below 1500 g (yes the internal zoom will be larger than existing), I think that would be a big winner.
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I was fortunate to be able to upgrade to the A1 and 300/2.8 with 1.4 TC and that is just in another league. This combo is still pixel-level-sharp at 50 MP. (Also the no-blackout EVF is another game changer).
The downside is that now I'm carrying 2 lenses on 2 bodies: the 70-200 and the 300, and I have a 1.4x TC on both of them. So I basically have 100-420 mm f/4. At least each one by itself isn't too heavy.