Excellent shot, but, in spite of all the foregoing comments, I feel it is not as sharp as it could be with this camera-lens combination.
May I suggest that you ditch the teleconverter, they do nothing at all for image quality - just give you a bigger image that is probably not as good a one taken without, enlarged to the same size and cropped. You've got 18Mpx to play with so there's no shortage of resolution.
ISO 1000, f5.6 and 1/640th - are you sure about that? Must have been a very dark day.
How about dropping the ISO, your camera's noise rises quite noticeably beyond ISO400, and if the light will allow stopping down to f8 or f11 - this lens can take it. That'll bring you down to around 1/250th which should be fast enough for a goldfinch as they tend to stay reasonably still when perched or eating. Using RAW you can always crank the exposure back up again later in post processing without any of the grim penatlies of a high ISO.
This should give you a very much improved image with needle sharp focus from the tip of the tail to tip of the beak. It would also resolve a little more of the background which is a bit amorphous at the moment.
In short, a fine image that deserves a little more detail.