600w is about the perfect amount of light for that space. I would have gone 400w and added in supplemental light to make up the difference myself, but you can do that later when you upgrade to a 4x4 or bigger. You should be able to run that at 100% for nice results. What is your fan/ducting? If you aren't moving the air at least to a different room we found your problem.
You need 1 inline fan, 1 carbon filter, and 1 duct leading outside or to another room where it will mix with a large volume of climate controlled air before heading back into the grow room. Or you need to seal it up and install AC and humidity control.
Save money, lol, $110 should cover it. Including fan, filter and duct, that is 4" and should be plenty big for your tent.
This is correct. Run the bulb at the wattage the mfr intended. Running at fifty percent gives much less than half the light pressure and spectrum suffers. Worse, it will wear out the lamp itself prematurely.