I think the plastic wears out with use and that allows vertical flex in the spring and then they start breaking bits off and eventually snap in the middle so they're no longer a spring. I've done it on three pairs of shoes so far so it's not specific to my cheapie pair.
Different types of carbon: SLX is the lighter and more expensive of the two.
I don't know anyone who goes throw C-springs like you so I couldn't say , but I'm not aware that the actual hardware has changed.
I switched a while back, as the price difference was the price of a pair of Keep-on Kovers which I went through (ie lost) like, er... C-springs.
Academic anyway, as I read elsewhere that SP have shelved the non-aero version...