I just got a Flux and have been running it on 4S, but shopping for a 6S setup. I've been using 5000mah 25C NeuEnergy lipos and haven't seen any probs and don't expect too - everything, motor, esc and batts has been barely warm after a 20+ min run.
I was considering Zippys for a 6S power source, but will give it a second glance after reading this. I've not seen this Zippy concern mentioned on Castle's thread on RCMonster, which is surprising since there are many that run Zippys on RCM.