It's a theory that I've been inquisitive about for a little while now, and I wouldn't be so quick to write it off. Simply because, going by spectrum and other formula's probably falls short of whats going on with the overall scheme of things. I think it's going to one of those hard ones to prove.
I've got no proof myself, but if you feel the light on your skin that comes off HPS, it resembles that more closely of what the afternoon Sun produces. And with my outdoor grows over the years, most of the plants growth appears to happen in the afternoon Sun, more so than morning. Which leads me to suspect that a crop could potentially benefit from HPS light in the last 4 hours of each day in 12/12 light cycle.
I'm in no way suggesting there's big gains to be had. But more like it could be a beneficial inclusion that could add a bit to the overall end product. Which leads me to believe there's no perfect formula out there yet, to which we can get an accurate measurement of the theory.