Sorry. I thought you were talking about lighting for the 55. That should be enough as far as light, but keep in mind that the light won't reach the bottom of a "tall" as easily as it would a standard 20. That being said, you will probably have closer to the equivalent of 3-4 watts/gallon.
I would think you'd be ok with some low and/or medium light soft corals.
Anyone else have an opinion?