I wasn't talking about the heat to your tank, but the heat to the light. You are going to be doubling the heat to the fixture. The PC ballast are probably mounted internally and they would definitely start to cook. I have done something similar to what you want to do but I added an additional fan to dissipate the heat. I changed my Orbit light from 4-65w pc's to 2 - 175 SE MH Reefflux and it did a great job on my 75g. One 400 would not give you even light, if you go up in watts, it would be better to go with 2-250's for a better distribution. Keep us informed I am sure a lot of people on this site are thinking the same thing you are in going up in wattage.