If your tent is 6 feet high, you have about 3 feet of usable grow space, from the surface of the medium to the bottom of the HPS. If the HPS isn't in a cool tube, you probably won't be able to get it less than 10"-12" from the plant tops. So you have two issues:
1. You're already getting close to running out of headspace, once your plants go through their growth spurt when you go to 12/12, and
2. Heat. You will probably need better ventilation, and/or more airflow over your HPS. My last grow was a major PITA because I didn't plan for headspace, and I had to supercrop and LST every day toward the end of bloom, and I'm sure it hurt my yield. You really don't want that problem.
So, yeah, you probably don't want to wait too much longer to go to 12/12, and you're almost certainly going to have to improve ventilation to maintain temperature.
Just my $.02 worth. Peace!