I run a pair of titanium 250w heaters in my 170 w/55g sump. No problems. However I have a baseboard heater in the fishroom that keeps the temp decent when the woodstove is not running.
Mine uses pendants, so has an open top also. I would thing 550watts should heat your tank, esp. in the day with the 400w halides on. Is the room that much colder than the rest of the house?