OK, I went and splurged.
Lots of back and forthwith some familiar LPs (B"P"B's Master and Everyone, and Bill Callahan's newest LP), listening for differences. Well, the harder you look, the more you find. Which is to say, there was no great difference. I think I found a little bit more texture to the guitar strings on some songs with the white belt, but better focus with the black (really hard to tell for certain though). However, while switching back and forth between belts, I noticed something: the stiffer white belt seemed to some inconsistencies, causing it to wobble on the sub-platter. Possibly it will need to break in, but it seems strange that such an inconsistency should exist in such an expensive 'upgrade'.
System:
Audio-Technica atmm440MLa
Rega P3 2000
Groovetracer Acrylic Platter
Cyrus 2 integrated amp
Nordost Flatline
VonSchweikert VS1
