I remember back when ioun stones of any kind were amazing to find. That was back when I would have to wait quite a few levels to pick up good ability bonus items. Higher ability bonus items are available earlier than they used to be, so most of the ioun stones seem a bit less special or less useful. Perhaps the ML5 upgraded ability bonus ioun stones could be revised to give a slightly larger bonus, or perhaps even have some other interesting effect added to them? The unupgraded ioun stones could probably have some minor benefit added.

PLIS and VPIS probably don't need anything added to them, since they both still seem quite relevant.