My opinion(not like it fuckin matters), the sub is rated at what it's rated at for a reason. We all know the coils, even though only 2.5", can take a ton of heat and abuse. Double the rated power for 5 months, I say you did a good job. Some may have lasted longer, some shorter.
Heres the rub: You can't really shit talk the sub for busting tinsel leads when you ran double the RMS rated power at the sub. You can call Sundown and discuss your dissatisfaction, my bet is they'll say," Well, we rated the 2.5" coiled sub at 600W RMS, AND it will take that all day every day. You chose to run 1200+ Watts to each? You should have bought a Z series".