Go with 8S! I'm loving my Savage Flux on 8S. I just recently tried the Zippy 4S 4000mAH 25C packs. They fit in the Flux boxes and have been working great. All for under $200 bucks for both packs. The 1521 stays nice and cool. BTW, the Cen diffs have been holding up great, as has the Flux tranny.
That is good to know, you are running a 1y right? That must be potent.