For bashing, that would be a much better setup; you have to remember that castle states a 2p setup MUST be used with the MMM/ Flux, otherwise its a warrenty void issue (voltage drops under load too much causing ripple currents and whatnot that the esc doesnt like).
Speed wise, if 4s a123 is fine, then I would use 4s2p for better runtime and increased punch, but 5s2p is nice as its equals 4s lipo, which is perfect for a 2200kv motor.
All personal preference at the end of the day though..
