I yielded 114 Oz bone dry weight 44.5 plants under 5 * 600W HPS with no CO2, just Advanced Nutrients products during the first week of February 2007. Temperature was not an Issue because it was the middle of winter. Half the room was G-13, just under half was Chronic with 4 Sensi Star. I have pics. The room has/had never been so rammed with plants. The G15 were over 5' tall in their pots. The Chronic and Stars ranged between 18 and 24 inches.

It is possible to exceed 1gram per watt:

114 ounce = 3 231.845 636 25 gram
5 * 600 = 3000W

Difference 231.85g or 108% efficient in terms of 1g per W