I use a 2X on my 70-2002.8IS and really enjoy it though it certainly isn't perfect. As said above I think to give it the best shot I have to shoot RAW and manual. RAW gives a better opportunityto pull out some dark shadows that I can't seem to do in jpeg.
I thought your shots were fine, trust me, I am no pro at this. To me, it 'looks' as if you were auto exposing on the overall background leaving some of the players a bit underexposed. If I knew more about retrieving EXIF info on postings I might be of more help. I have an older 70-300 that I sometimes use alongside the faster zoom and the difference in detail is startling at equivalent apertures. If I get any good shots with high sun I consider myself very lucky.

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