erab, 600W in an 85cm x 50cm is overkill for sure, but it certainly isnt a bad thing, just means you could grow more than 4 plants, however in that space you can really only fit 2 plants, maybe 4 if you lollipop them. I'm using a 50 deep and 130 wide grow space and have comfortably finished 2 plants in there (topped and LST) and could have made 1 more fit. This time round I'm doing 2 again, and doing a scrog to fill up more of the horizontal space, however with your smaller space and higher powered lights (I'm just running CFLs) you will be able to yield a bit more bud via vertical growth I believe.
Your major concern is heat, and 40deg after 2hrs is not a good sign. If it has a cool tube then install ducting to vent that air outside the cabinet, and have the intake ducting sourcing air from the coolest place possible (ie under the cab, in a dark/cool area, next to an AC vent). You'll also want a passive inlet and a fan powered exhaust to move the air in your grow space, and a lil fan blowing onto/accross your plants. Bunnings have awesome small fans (10-15cm) for $12, they are very quiet, compact, and have a decent output.
Anyways I'm rambling, this is what happens when I havent smoked all day!