Upload & Sell: Off
I've used my 500/4 with a variety of Canon bodies and with a variety of 1.4x and 2x converters, and I've never been a big fan of using this lens with a 2x.
While I've had some very good results using the 500/4 and a 2x with relatively static subjects, my satisfaction with the results has diminished quickly if I'm trying to shoot moving subjects. Further, image quality degradation is significantly greater with a 2x than it is with a 1.4x. That image quality degradation doesn't show up to be as big when using a low pixel density camera, such as a 1D Mark II, but, I've found that the loss in image quality manifests itself to a much greater extent with a high pixel density camera like the 7D. With that higher pixel density, any image imperfections, such as those caused by even minor camera shake or subject movement, are magnified to a much greater extent than they would be with a low pixel density camera like the 1D Mark II. Thus, I'm less inclined to be happy with the results using a 2x with a 7D than I would be using it with an older camera with lower pixel density.
Additionally, using a 500/4 with a 2x on the 7D gives you the field of view of a 1600 mm lens on a full frame camera. I find that, with that field of view, I feel like I'm looking through a soda straw, and I have a much more difficult time picking up the subject initially and tracking any movement of the subject. Further, in any situation where a field of view that is that narrow might be appropriate, the distance between the camera and the subject is likely to be great enough that atmospheric imperfections are going to further degrade the image. Thus, I have no interest in even trying to shoot subjects at those distances.
With regard to the version II and version III extenders, I know that I am in the minority here, but I consider my purchase of the version III extenders to have been money not well spent. While I have no doubt that those who have done comparison testing have found (very minor) image quality differences, in real world usage, I have found not even the smallest level of difference that will show up in a print. To me, the only significant difference is a slightly beefed up mount on the version III extenders, and that improved mount might lead to better long term performance of the converter.