This shot was very unplanned/no time to prepare to take the shot really. I was capturing a Hawk in a tree, then decided to try and get some shots of this plane as it went overhead. I didn't expect them to even turn out, but they ended up being pretty decent. Normally I shoot at F/8 now to get the best overall sharpness. F/7.1 is still not bad.
However, I just did a test at 400,500 and 600. Even at 400 F/5.6, it's very very sharp. It noticeably softens from there to 600. But F/8 at 600 gives sharp results. F/9 might be a tad better, but then you lose more light. I'm going to test now and see if I can crop the 400 shot at 5.6, to match the 600, and see if it still appears sharper.