Having 2 or even 3 of those, linked, will probably be perfect for that size of tent. In fact, 3 would give you such a nice, scalable footprint of full spectrum light, that those could be ideal.Use curly CFLs for your babies, and then move them under one of the cobs, and as they get wider and taller, add another, then another, etc. You save a lot in utility bills, effort to cool your grow space,etc when you're not using ALL of your lights on your tiny little seedlings.
But I've been so impressed with the difference in quality with CMH, that I think that, rather than getting 2 more of the same thing, you might consider adding a 315CMH fixture to your arsenal of lights. The quality of finished buds has been far and away better than the my previous blurple and COB grows.