Same 600HPS here but in a 4x4 and regularly pull over 20oz with 4 and 5 plants in that space.
Soil/coco and Buddhas Tree Organicus and Ecothrive Charge and I've found that in my space I get around 5oz a plant if I do 4 plants and 4oz a plant if I do 5 plants.
Think the most I've had is just shy of 23oz form the above setup.
To be honest, it's like clockwork now and I expect nothing less.
I'd say there over 12oz there but just not 16oz.
Regardless, she's a corker.