I'm looking for an ENB I can use which only adds a depth of field (I think it's bokeh?) effect while avoiding all the other visual whizbang. Alternatively a minimalistic one with a few other features that can be easily turned off would work as well.

I would also be interested in knowing how much of a performance decrease it would be with just DoF alone.

Why don't you use the ENB Series proper rather than a preset? It sticks very close to vanilla on default settings. That's basically how I started out as I just wanted the detailed shadows/the vanilla banding of shadows gone and nothing else. By now it's tweaked and I use most of its features, but that's another story.

Try Dynavision. It's not really enb, but it just adds customisable depth of field effect.

It's just a matter of turning off the effects you don't want.

Install an ENB preset that you mostly like then open up the enbseries.ini, with notepad or something similar, and change the boolean values to what you want. Experiment a little and see what you like. The ENB file that defines the depth of field effect is enbeffectprepass.fx. If you want to try different ones just substitute that file.

You may want to try http://www.nexusmods.com/skyrim/mods/27601/?.