Nice thread for learning about the CRI.

I want to comment on the original question: Could I make an HPS lamp mimic an LPS lamp if I underdrive it?

Quick answer is no, simply because traditional HPS lamps have mercury (Hg) as a starter/buffer. If you examine the spectrum of an HPS lamp dosed with Hg being energized, the first few seconds will typically show the mercury spectrum (that's why most HPS lamps appear whiter at start) followed by the faint D-line pair of lines of sodium (at 589.0 & 589.6 nm). These lines continue to intensify, broaden and self-absorb, eventually producing the typical HPS spectrum. But when underdriving, the spectrum will always display the Hg lines prominently. So the spectrum is quite far from that of a SOX spectrum.

Now I don't have a mercury-free HPS lamp, so I can't say if the original desire to make it look like a SOX lamp is possible, but I'm guessing that it is.