i personally would not run 6s on my 8E(will be getting one soon)
the ESC may be rated for 6s, but thats gonna push it.

i have a pair of 2s 30c 5000mah lipos i got off ebay. $50 shipped for the pair. you do have to add plugs on them, but they are pretty good for the money.
the 5000mah means longer runtimes than the stock lipos.
just search AKKU on ebay and you will find them. i have the soft pack version, but for a few extra bucks you can get hard cased ones.