Did you buy a ballast? you need a ballast. Not just a bulb and reflector. And you MUST match the wattage of the ballast and bulb (ie - 600w bulb and 600w ballast...250w bulb and 250w ballast...) other wise it won't work at all or will burn your house down (seriously). If all you bought was the 600 watt bulb and have yet to buy a ballast, then you aren't in too bad of shape. Just eat the cost of the 600 bulb and get the 250w kit that will come with a ballast, bulb and reflector. Or see if you can return it. Lots of the hydro stores in the states are super cool about returns and customer service.

The added headroom is definitely good. But i still think a 600 is too much for that space for 2 reasons:

1. Still TONS of heat for a 23" x 19.5" cabinet.

2. The average 600w puts out around 95,000 lumens. According to the square inverse law, light loses half its intensity every 12 inches. So, if your light is 2 feet away from your plants because of heat, you are only looking at roughly 40,000 lumens...which still isn't bad, but move it up another foot and you're at about 20,000 lumens. The best 250w i've seen is around 33,000 lumens and you could put that shit within a foot of the canopy. Not to mention it'll use a 1/3 of the power and probably 1/10 of the heat.

as far as floor space in your cab, you have 3.1 square feet. I believe the ideal lighting for great growth is 5,000 lumens per square foot. So you are
looking at 15,500 lumens. Seeing that figure now makes me think you'd do just fine with a 150w ballast and 150w bulb.

I'd definitely like to hear from one of the pros on the forum. But that's what i've come to understand.

Givin' ya something to think about...
Skeet.