I do expect that you would need a medium or wider nib in order to get a real sense of shading out of the ink.

To your other concern, though, if anything I've found that Saddle Brown goes much darker when used in an F/EF nib. Certainly not to the point of being black, but dark enough that unless you were actually paying attention to the shade it doesn't immediately grab attention as being non-standard; to some people that defeats the point, but I find that sometimes (e.g. at work) I want to fly under the radar a bit while also knowing that I'm using 'my' colour.

It also looks amazing in notebooks that have a slightly ivory/off-white cast to the pages, where the overall sepia effect is something I really enjoy.