Now, you can do a 2s and 3s pack for 5s. I know you have a split pack chassis so it wouldn't be the best balance. So then 6s would be the logical choice.
http://www.allerc.com/product_info.p...oducts_id=4560
I would give those a shot!
http://hobbycity.com/hobbycity/store...0C_Lipoly_Pack
I have these packs and IMO they are under rated. Probably a 25C pack. These have lots of power and punch.