Arent you using a mag 12? You should be more than fine. Especially with the powerheads. Im currently using a mag 9.5 and NOW (after one year) of growth and more rocks I consider it under. I personally cant stand powerheads and plan to upgrade to get rid of them, but if you dont mine them then you should be more than ok.
Also Im pretty sure you use the 55g as your base turnover not your total volume because its this area that the corals will be living in. Total volume just adds to the stability of the tank.