Usually you want your prop to be spinning arround 30-35 k rpm so with more cells youd need to come down on the kv.I suggested a 40 mm diam motor minimum for that size hull.Higher kv doesnt mean more power.
Heres a handy calculator to help you work out what you need to get you in the ballpark.

http://www.radiocontrolinfo.com/RCca...Calculator.php