Upload & Sell: Off
I recearched this extensively, did my own testing with a 7d, 1d4, 5d2, 5d3, using extenders, various ISO's and searched for other tests.
1. Under LAB CONDITIONS if you are focal length limited (have to crop to the size of apsc or smaller, for the snesors of same generation, you are always better off with higher pixel density (obviously not crop factor, since 5d2 has same pixel density as the 20D even though 20d has 1.6x crop factor. its all about pixel density).
a: you wont be shooting in a lab
b: you are talking about densor designs of different generation, so 1dx may have better design
c: as you move up high iso, the gap becomes less, but the larger pixel density for the same final output size, will always do at least as well, many times better in detail
2: again for sensors of same generation (5d2 and 60D say) you are better off using they higher pixel density than a converter. the advantage here is slight. Note that those two will give you same pixels on duck (5d2+1.4x, is about same pixel on duck at 60d). of course, there is also great advantage to not having to deal with slower af of an extender.
3: In real life, many issues come in to the picture.
first, above and beyond everything else is technique. with long lens, high pixel density and cropping, you are looking at very fast shutter speeds and excellent technique to have pixel level sharpness anyway. if you cant achieve that, a slightly blurry image on 1dx, would not resolve any better on a 7d.
second, if you have to crop, I find having the viewfinder magnification of a crop sensor and framing properly, is helpful and a plus
third, all cameras are no created equaly. autofocus speed, accuracy, buffer size, general handling, can make a big difference in getting the shot, and getting a sharp shot.
fourth, low quality consumer lens is less able to take advantage of higher pixel density (advantage does not entirely go away, but becomes significantly less) so a simga 150-500 os at 500mm for example, will have far less advantage on a 7d over a 1dx, compared to say the 500 f4L lens.
Hope someone finds this helpful