I was testing a 2x TC on the moon. I wasn't too impressed with the results, so I went back out with the 1.4x TC. I took 8-9 shoots with each, and selected the best. I was able to resolve as much detail using the 1.4x TC. (100% crops)
1/320 sec, 400mm, F8, ISO400 (2x)
IMG_3361 fixed by
bkslopper, on Flickr
1/320 sec, 280mm, F5.6, ISO 200 (1.4x) I tried F8 and ISO 400 as well, but this was a bit better.)
IMG_3369 fixed by
bkslopper, on Flickr
I processed both in DPP, turned off noise reduction, used monochrome, +4 contrast, +5 highlights, -5 shadows, 5 sharpness, and set the EV at -1.5 (2x) and -1.67 (1.4x) to match as close as possible. I'm guessing a 15Mp crop sensor plus a 2x TC is beyond what the lens can deliver. I manual focused with the live view zoomed in all the way.