I have a four bulb box and with 100w bulbs, I had to bury it in blankets and it took a very long time to get 150ish. That was in a fairly cool environment. I now have 200w bulbs and I can reach 165 in very short order. Factoring in the room I am now in is much warmer, the time frame difference is significant.
So think about how long it takes to reach temp in relation to how well it will hold.