what are you using for flowering?
flouros can be cheaper than a hid for vegging purposes all you really need is enough light to keep the plants from stretching and have decent compact growth and flouros can do a decent job of that.
would probably make that a flowering box and hang a couple of 250 hps in it and use your existing box for vegging and you could probably get by with alot less flouros if you used your existing box for veg purposes alone
you should have no trouble keeping your plants 8 inches from the cool tube with a 250 and probably be able to get within a 3-4 inches if you have decent ventilation without burning the plant tops.
a hid isnt 100 percent efficient and you would be pulling more than 500 watts but it would be less than the 800 watts of cfl and if you went with hps you would be well over 50,000 lumens