sandwashed

Interesting to compare these different treatments of the same sky. I think that this is my favourite version, with the more saturated blue sky contrasting well with the yellowish clouds.

That's a difficult one! However, in this case I think the answer is a combination of two things. First, any fiercely backlit or silhouetted feature is always going to test a lens to the utmost, since flare will add to any other degradations that the lens might already possess. If you add to that the fact that no lens is going to perform at its best at full aperture (f4 in this case) then you're never going to get perfect crispness in a shot like this.
However, I think you could have done one thing differently. You shot at 1/500th at f4. That means you could equally have shot at 1/125th at f8, and that shutter speed would still have been fast enough to take the shot hand-held, but at f8 your lens would have been performing much nearer its maximum capability.
Having said all that, I don't think the slight softness in the trees really spoils your shot very much. The trees are not the main point of your beautiful shot of the sky, so a little lack of tree detail doesn't really matter, it seems to me!
Hope this helps!

I favor this one too, I think because of the "nice" blue of the sky ...