What is 'hard data' in grow lights? it seems to me there is no agreed metric that encompasses all types of lights.
Add in that different strains appear to do better with different spectra, and high PAR lights can be built that are useless as grow lights, making it seem to blow any idea of a standard metric right out of the water.
Perhaps we need to step back and take a different approach. That would be to compare the performance of a standard plant, grown in standard conditions with different lights, to the same standard plant grown in optimal conditions out in the sun.