As always, I come in late in a discussion....
2,5 gramms per watt is possible.
If you veg forever, grow huge plants and flower a couple of months, on a big space and also consider fluffy weed as 'good' weed.
For a good comparison you need the following things:
Space x time x watt x gramm
I hardly saw anyone who is doing 5 or 6 yields a year do more then 1 / 1.1 / 1.2 gramm per watt on a regular basis.
I know that with Led more gramms per watt are possible.
But even then I see yields that look so horrible, with all kinds of fluffy shit, and even the smallest nuggets which they count in.
If you grow from clones + 2 weeks veg + 8 weeks flowering in a space 4 x 4 under 1 x 600 W HPS, then you are an excellent grower if you yield 600 gramms of dense buds. 500 gramms = still very good. 700 gramms = superb, 800 gramms = wow, 900 gramms = buy a new scale.