I'm pretty sure that Ifrit enraging when you have a lot of dps on him is just an urban legend. You guys probably just din't have enough dps to kill the nails because the summoners were wasting time on Ifrit just to inflate their DPS.

In my party sometimes we kill it just after hellfire before he can jump to do hes rushing attack and we have never experienced that issue, and hes really low on health when that happens.