If you have room you may want to consider getting the VHOs in the name of saving electricity and life span of your MH bulb. You can cut your MH back to 6 or 8 hours a day and still get 12 of veiwing with the actinic. Another added bonus is the ability to get full actinic veiwing which is cool in its own right.
I have 500w of 14k phoenix and 330w of URI VHO actinic. When the MH are on and I turn off the VHOs I can barely tell the difference at first glance.
IMO your 400 would probably completely wash out any VHOs you put over the tank.