Perhaps what you're seeing (the plumage looks OK to me) is due to the use of the TC17EII...the IQ on the 500 isn't quite as refined as the TC14EII renders.
Nice work Jim. Excellent composition and great perch for this handsome bird. I might take a bit of the top and lose the three prongs on the bottom of the perch.
Looks good to me !!! Jim you might want to take a peak at a dragonfly I posted in macro (recent) it is with the 600 and 2.0X Those converters work a lot better than we give them credit !!!
The main difference I've seen while using the 1.7X vs the 1.4X is the AF speed Does degrade the speed but fully usable.
I like how the bird pops out of the bg sort of like 3D Very fine image !!!