Went for a walk with spouse at a nearby national wildlife refuge ... tried out the sigma 2X teleconverter on a d7000 with the sigma 120-300 OS. I love this lens with no tele and I was quite impressed with the 2X. (obviously it is no 600 VR)
These are all 100% crops from about 50-80 yards away. It is not a bad combo (AF fine tune is at -20 and it probably needs to be -22 through -25), but definitely not a prime. It is slightly soft (not too bad on screen when compared side by side in test shots - I have not compared real world shots yet - to the 600 VR. However, it is obvious when viewing the files at 1:1 resolution), but I think the contrast and dynamic range is really what suffers the most.