Over a 55g, there would be 2 metal halides (one over each half) and neither would be near enough to the center brace to melt it. Unless restricted to a short canopy, halides (or any light for that matter) can be placed at whatever height is needed for them above the tank. Increased height off the tank comes with advantages and disadvantages, it's simply a matter of balancing them to get the results you desire.
Nonetheless, my mentioning of metal halides was more for reasons of why watts is a poor indicator of light quality. Although, there are anemones that would do far better under MHs than the number of T5s that can conceivably be placed over a tank as narrow as a 55g.