I wouldn't expect anything higher than Vive Pro resolution, because the only way the leaked specs can support HP Reverb resolution is if they include something like foveated rendering. Which isn't very good to expect, as it's likely setting yourself up for disappointment.

However, you are putting way, way too much emphasis on resolution. HP Reverb resolution with normal VR optics will looks way worse both in clarity and SDE than Vive Pro resolution with really good optics. Given the double fresnel lenses being a selling point, it's reasonably safe to expect Valve Index to have good optics. With good optics and Vive Pro resolution, the Index could offer a PPD higher than any other consumer HMD - including the HP Reberb, which probably can't match Index's optics.