Recently @Joe was doing some digging on 'actual' cache hit rates of Restbase services by investigating webrequest logs.
Unfortunately there's not a definitive way to tell what subset of traffic against these (or anything else in webrequest) comes from logged-in users*. Seeing an X-Cache header with two passes probably indicates that, but I don't think that's guaranteed.
It seems like it would be useful to add this notion as another subfield of X-Analytics.
*: By "logged-in users" I mean the CDN's rather loose idea of having a Session|Token cookie