Every time I try this warning, it finds a few bugs (I think about 4 total in Chromium so far), with 0 false positives. I couldn't measure any build performance degradation when turning it on; I tried building clang itself with and without the warning, and a small-ish target in chromium (1.6k TUs).
It should be in -Wall, and possibly even make its way into a default warning at some point.