Using a 2x converter is a hit-and-miss affair. Manual focusing is the main challenge, and this is not helped by the way the converter turns f5.6 (at 300 mm) to f11. Less light makes it harder to see if the desired object really is in focus or not.
I photographed a Darter and a Little Pied Cormorant this afternoon, the former at 26o mm and the latter at 300 mm. The darter was in better light than the cormorant. The differences in focus and clarity are obvious (I had to be a little heavy-handed in sharpening the image of the cormorant).