This is a shot right after I bought then 500mm II.
The grass isn't to bad.
20150117-CQ2A2535 by hdnitehawk01, on Flickr

With the old 500mm it would have been worse, kind of like the rail shot you posted.
This is the closest one to showing what I am talking about that I had on flicker,
Cardinal 1 by hdnitehawk01, on Flickr

The tiny branches are close and are unpleasing to my eye. It is a separation issue, but the new 500mm handles it better.

I will be curious about your perception of the 500mm.