I just replaced my t5's with a 400watt.... I kept the same time and for the last two weeks no ill effects although I noticed that my birdsnest went from a darker brown to a lighter green.

Of course my bulb is used, and I started with a spider reflector. Now I've got a lumenarc. I'm going to keep it as is and next week I will flip that switch on the ballast for extra juice. Another week and I will put in a new bulb.