I had ran a 315w cmh in a 3x3x7ft tent with a single flowering autoflower plant, Northern Lights Blue. It fetched me a yield of between 3 and 4 ounces. I was shocked that I pulled so much off of an untrained, untopped, christmas tree shaped plant. I haven't tried to recreate that grow again, I usually flower my plants in a 6.5x6.5x6.5ft tent. I think it worked out so well because the tight space allowed buds that weren't recieving direct light were still able to recieve a great deal of indirect reflected light. Just be sure to grab a 3100k spectrum bulb if you decide to roll with that...good luck buddy.
Oh and I about forgot, for that grow I used the Vivosun reflector and Philips 3100k bulb. If I could have a do-over, I would've went with the Sun System Neutron Reflector Remote Ballast setup. It's a much smaller and compact design, and the remote ballast allows for you to situate your ballast outside of your grow tent rather than it being a part of your fixture which in turn, creates more heat in your canopy.