Honestly I'm surprised 1.5 is perceived as an outlandish claim at this point in the game. There are factors that limit yield, PH to high or low, to many nutes, not enough nutes, root rot, too much water, not enough water, humidity too high/low, canopy temp to high/low, insufficient air exchange, etc. but when everything is dialed in there's no reason why people shouldn't be hitting those numbers from mid to high yield strains if the lamp output provides at least .5 par watts per watt.
I went from .7 GPW under CFL to 1.25 under 35% efficient Vero 18, just like that. Results under gen5 Vero 29 at .7 amps was always between 1.45 and 1.68 GPW over various strains. I had some experience under my belt at that point but I'm not a master grower or anything. Perhaps my expectations are unrealistic