I picked up a used 2.0III extender and tried it last night for the first time. Nothing scientific here and a distant subject, probably 300'. I expect that the 2.0 will only be useful in good light and probably questionable for bif but it worked better than I expected on this test run. Shot from a bean bag, cropped to same dimensions and the same basic LR edits on each.
1 - 500mm, 1/2000, f4, auto ISO180
2 - 500 + 1.4, 1/2000, f6.3, auto ISO-500
3 - 500 + 2.0, 1/2000, f9, auto ISO 1400