Sig Red is more valuable because whole chunks of time go by without it on the market, and certain people "have" to have it. I can say this, having watched, and bid, prices north of 89k, toward the end of Summer 2014. Then, having had a chance to evaluate one more closely, at least with the Model S there is a bigger story behind that paint. It's deep. Like .4mm thick, which I'm convinced is part of the candy apple luster that the color gives off. I'm pretty sure the other metallics are uniformly .2mm.
What I don't know, is if the MX sig reds are robot painted, versus the hand sprayed MS cars. If so, and if to the new thinner depth, I bet a difference can be told by putting them side by side. I am not a wonky person when it comes to paint, but became one in a recent experience that ended well.
Only my belief, but I think the bigger issue to sig red cars fetching a premium is that Tesla buyers buy from Tesla, who doesn't see it this way. They are such a "huge player" in the market for their own cars, that it's hard to see value deviate much from where they put it. The post-CPO market could be different, if Tesla becomes less interested in selling at lower price points.