I think Weber prides themselves more on the sound and construction than the efficiency of their speakers, lest they help encourage people to shop just based on efficiency specs. Buried on their site are a couple graphs of frequency response, from which you could guesstimate the overall efficiency, but they're old and possibly out of date.
I believe the Signature series is a little less efficient than their bigger-magnet lines.
I've heard Weber sigs (and higher-end Webers). I've heard Jensen Mods and higher-end Jensens. I've heard Celestion AlNiCo Blues and V30s. Trouble is, I've never really given the time to A/B speakers. I play a 12-watt amp, and would definitely like something that gives me volume and later (but sweet and musical) breakup.
I don't know any source for the efficiency of Weber speakers. But I do know that when I compared them to the Jensen P10R, the Webers were noticeably louder, all other things being equal. That suggests they're at least 3 dBa more efficient at those mid range frequencies that my ear hears best.