Grams per watt is, frankly, the most useless measure there is. It all depends on how many plants you're growing, for how long, and the amount of light, and many other variables that are so out there that it's simply not possible to count it as a reliable measure.
Take my example, for instance. I grow two plants at once one time per year under a 600 watt HPS. I usually net between 4 and 6 ounces per plant. Let's say I get 5 from each. That's 10 ounces. That's 284 grams. That's .47 grams per ounce.
If I just grow two more plants, which is very easily doable, that nets me 20 ounces which equates to 567 grams which is .945 grams per watt.
If I go back and grow 3 plants but run a scrog on them, I can easily get 8 to 10 ounces per plant. That scenario leaves me with up to 30 ounces, which nets you 851 grams, which runs it up to 1.41 grams per ounce.
And that's still not a maximum effort.
There are so many different possibilities, strains of high yeild plants, growing methods and lighting combinations that grams per watt is, sorry to say, a completely and totally useless measure.