Looking for a narrow-cage 16T capacity FD I'm putting a 46x30 chainring with a pretty narrow tread on a frame. On the advice of my LBS I got a Sun XCD double FD, but I'm starting to think it's not right for this application. When I have the derailleur on the big ring and far enough out to avoid rubbing, there's razor-thin clearance between the outer plate and the crank arm. And that's just on the stand -- I'm worried that once I'm on it and things are flexing, the crank is going to hit the cage.

I think a narrow-cage derailleur, and maybe one that is designed for smaller chainrings (the Sun is listed at max 56T) will be a better fit. Does anyone have any recommendations?
There will be only very small differences in the widths different FDs (assuming you are talking about the inside to outside dimension). One that is designed to be minimum sized so that it provides a best fit for 46t large chainrings is Shimano's CX-70, intended for cyclo-cross:
http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/cyclocross/cx70/product.-code-FD-CX70-B.-type-.fd_road.html
It does have a 16t capacity. It is also compatible with the bar ends (which don't index the front anyway).
I have purchased one of these to use with a similar chainring setup and 105 dual-STI brifters, but I have not yet installed it.