Ok, I just looked back at what I wrote... when I said to add some more CFL bulbs with a 2700k color temp, I meant that you want to stick with primarily 5500k+ CFL's during vegging, but a few 2700k bulbs mixed in will benefit. When you start flowering, swap to the 600w HPS. Do the best you can to control the temp once you start using the HPS.
5500k+ and MH bulbs = blue light spectrum (primarily for vegging)
2700k and HPS bulbs = red light spectrum (primarily for flowering)
Mixing has great benefits.
Vegging light ratio (red:blue) = 1:3
Flowering light ratio (red:blue) = 2:1
I say 1:3 for vegging because some people argue that the plant absorbs such a small amount of red light during vegging that 2700k isn't necessary at all. If you have all 5500k full spectrum CFLs for vegging, then you probably have enough red light to suffice. Personally, I mix in a 2700k CFL during vegging.