Aside from beoing outside of what is accepted as the spectrum required for photosynthesis for our critters, I think most hologens fall short of 4000K, they are terribly inefficient, something around 65%. I need to do a bit of research to verify these numbers for certain. But if these assumptions hold true you could run a pair of 400w halides for the same amound of energy consumed by a single 500w halogen. But their price can make them look attractive if you can find a way to dissapate the heat created, especially over a 6+ hour photoperiod.

But if you give it a shot, throw up some pics and let us know how things pan out, espcially any long term results.

There have been successful tanks running nothing but the 3000K mercury vapor lighting ( < $30 ), think Peregrinus still has a 55g up and running with them.

Experimentation is always a great thing, though the results..... generally interesting.