To answer the OP -- the Z v.4 line has been known to handle ~2x or so of it's rated power.
A thermal failure is unlikely in most applications -- you would be more likely to damage the lead-wire via clipping or perhaps even damage the spider itself physically. In one of our test bench setups here (Jovanni's Golf) on a 3500D @ 0.5 ohm we saw these type of things before thermal failure. He did have a massive 3 cubic foot box for one 12" woofer, though, so a more reasonable box would largely eliminate the possibility of physical spider damage (2.25 or so seems to be pretty ideal in our testing so far). So just keep an eye on clipping and you can run a ton of power on them =)