Sorry, not a member, but from R.A.R.E. up north, just scanning your for sale stuffs, and saw this post, thought I could chime in.
I am running the tek 6X54w T5 fixture on my 55 gallon. As far as lighting I have never had a problem with anything, other then that the 6X54w fixture is actually a bit wider then my 55 gallon.
I think a 4X54w would be awesome on a 55 gallon.
I am hopefully going to set up a 48 inch 120 gallon tank in the near future, and then I can move my 6X54 to it, and put a 4X54 on my 55 gallon.
T5's don't offer the shimmer effect that metal halides do. But from what I have experienced in the couple months I have had mine is a little boost in most soft coral and LPS growth, I am in the middle of attempting SPS and having bad luck, but I think it is my water params, not the lights by any means, my Bali slimer probablly grew a 1/4 inch in the two months I had it, then it got knocked into the sand, and I didn't noticed it for a couple days...
So there is my story of my T5s. I got my fixture from Reefgeek.com and the guy that runs that was great about helping me with the choices in lights in it, because there are a lot of choices for bulbs.