One reason why my 600mm f4 is gathering dust

I looked at the images from first post, not later processings, so if that makes a difference, sorry. However, as a 100-300 user myself (On a GH2 if that makes a difference), I think some of these photos suffer from lack of sharpness due to long shutter time and not due to quality of the lens.
Personally, I prefer to increase the ISO rather than have shaking in my images, your opinion may differ. If I have to compromise on shutter speed (and preferably even if I don't have to compromise) I will shoot burst of 2-3 pictures and 2-3 bursts per motive, and often find even at 1/400 that one of a series is better (more sharp) than the rest.

Niels