Back in the day of single-ended bulbs only, 3x400 watt MH bulbs is not unheard of. In fact, a LFS has a 10-year old 200 gallon display with 4x 400 bulbs and 4x VHO URI actinics. They are getting some serious growth and color with no signs of bleaching. However, with DE lamps on the market, I'd go with 250-watt DE on a M80 pulse ballast (Radium) or Ushio/Iwasaki version which requires an M58 ballasts. The 250-watt DE allegedly producen nearly as much PAR as a 400 watt bulb.
Note: there are few electronic ballast that drive DE lamps. Icecap offered one in the 250 watt range but it is discontinued. There are some hydroponic 250-watt ballasts that should be able to drive a DE ballast but I'm not sure. Most folks are going back non-electronic as they are more reliable. 15 years ago I ran Blue-line 400-wattt electronic ballasts. They lasted 5-years before dying on me.