IMO, a dual Reef Sun system would be ok for a 55g tank. The unit is 39" long but MH are usually placed 2' apart anyway. With the built in reflector I can't see a problem with it.
Dual 175w MH IMO is pretty good for a 55g. You may want some VHOs or PC to go along with it.
Something else, the unit I found came with 5500k bulbs... Something to think about, you may want to upgrade them.
All that said, there are cheaper units out there. The sun reef is a pendant mount unit. If you plan to put it into a canopy then there are some retrofits that would be just as effective. If you plan to hang it above the tank it looks like a nice setup.