I'm late to complain, but using this for a couple hours I find it a bit annoying with long C++ signatures. I got many cases where the popup now spans most of my screen width, making the thing a bit hard to use quickly. Also, although useful, the signature is so big and prominent that the filename is almost buried and harder to find; and many times I have the same signature because it's just all implementations of an interface, so the signature itself is not that interesting in that case.
I didn't test the tooltip version, but I'd guess I'd like it better, what was the reasons for you to abandon it? Maybe we could find a middle ground :)
 