I've got a 2x2x2metre tent, and 3x600W HPS lights (1800W total).
I had heaps of problems with heat.
I switched all three lights to cool tubes, all connected together with one fan hooked up to extract, and they work unbelievably well. No more heat problems. In fact some days its too cool in there.
Pics in my grow journal if you want to checkout my setup.