I think the gains from 500 to 800 are substantial. I totally understand and like 800+ myself.
Just thinking though, the super efficient garden...
3590s at .35 amps and 500 PPFD. 70% efficiency would only need 350 watts to achieve PPFD of 500 in a 5x5... 15% more light than a 600w HPS plus less reflection loss. I won't guess at what GPW might be but I think such scenarios will push 3 GPW before high PPFD does.
I'm not crazy enough to build it, but several posters here are... they just need to quit messing with soil and go hydro and 3 GPW will become a reality.