I'm with Tom on cool tubes etc, my hps bangs em out but I still want it out of the tent to balance the temps across the tent... Check that the 600w hps bulb is OK to be dimmed.. I was under the impression that if you swap it down to 400w you need to put a 400w bulb in it as the bulbs are designed to run at the designated level... I may be out of date with this though... I ran 30 600 w lamps for a while about 25 years ago... But lots has changed and I got old and forgetful loli think your doing a stellar job working with what you have also when i used to grow with hps i had a dimmable ballast and a cool tube and it made so much difference in the summer months also regarding the cfl i always found them great for early veg
Your plants are looking bloody good though @THE BEAR