There are really no pros to using the amount of CFLs that would equal a 600w HPS. For vegetaive growth, CFLs are a better option than HPS because you dont need the intensity and the heat and electricity are much less.
You dont need 600w worth of CFLs for veg if youre using the same wattage HPS for flowering. A T5 or even T8 fixture would be even better than spiral CFL's.
I dont understand what you mean by "mount them all on one sheet of plywood" If you use an HPS, get a reflector hood for it and hang it from the ceiling.