That's a fantastic Q re: the ceiling after which there is no additional gain.
I can't give a specific answer but I CAN say that the best youcan hope for is to find what your limiting factor for growth would be, and work on improving it. It is USUALLY light for indoor growers. Once you have lots of light, you will want to look at other factors... soil moisture, atmospheric CO2, carbon availability in the medium, and of course your nutrient load. This is why many growers who use CO2 are not getting as much additional oomph from it as they could.

600w light is more efficient in lpw than a 1000. That's one thing to think about.

Your big thing is going to be temps when you really crank up the light.