If you can support using a 600 hps then do so. Ok, some folks are saying it's overkill of lighting but then again we all also know that strong light intensity equals large/dense buds which at the end of the day is what we are all trying to achieve.
IMO, with a cab of that size the main issue here is not the light intensity but how to deal with the heat generated so as not to fry your ladies. IMO this is far more important than anything else, a 400hps pumps out a lot of heat, a 600hps as much as we love them gets even hotter and your not just talking a couple of computer fans to control temps.
We would all love 3 1000hps's in a row but with out proper,super efficient,and well thought ventilation it just isnt poss.
My suggestion, start small 250hps, see what happens with plants/temps and the other variables and progress from there.
Grow on.