This brochure is probably a little more enlightening on the WAGO Cage Clamp system. Particularly page 2, which shows how the installation process works.
Mike,
What kind of currents are we talking about here? As I understand the system, all/most of the current should be flowing through the copper current bar. The spring steel clip shouldn't see much current, given the relative sizes and resistivities of the clip and current bar. So I don't really see a mechanism for the extreme kind of heating needed to take the set out of the spring steel, save a rather poor installation.