Any TC advice on D7100 with 300mm f/4?

Indeed.

I am not quite sure what to make of the crop mode on the D7100. The increased FPS and smaller file size is useful but the crop itself is just a crop. Still, at least it is 15mp+. And the lack of AA filter could/should allow a harder crop.

The Panasonic G series have a similar crop (jpeg only) when using M and S image sizes which I never found useful, partly because I usually shoot raw and partly because a distant subject is easier to frame without it followed by cropping in post.