It *may* look right over a 90g tank. Halides look best when placed about every 24". You may end up with a dark area on the sides. Of course with that light, you could place it higher above the tank.
As far as a cost to run, assuming a 0.09/KWh rate, and a 10 hour lighting cycle, the (2) lamp 400w system would cost about $262/year to operate. A dual 250w system would cost about $165 in that same time period. So that's $100 per year you are saving in electricity. In 3 years, you'd save enough to buy a properly sized fixture. Don't forget though, you can't just change the lamps, you need to change the ballasts as well.