Hi John,
you can prove your conclusion by using a pic with out of gamut colors, like “riwodot colortest I / II” → download link here …
Yes, I’ve seen a difference when moving PL’s main screen to the sRGB screen (= setup to sRGB) and then toggling SP (= set to ARGB) on ↔ off. Yet, it is more affecting the brilliance & saturation because the sRGB screen then limited to that color space simply cuts off what cannot be reproduced, while when softproofing ‘correctly’ with sRGB the SP functionality takes care to map the out of gamut colors into the reproducible color space.