If the bike had been built for Boyer, value is increased..that's just common-sense. If he was the original owner but it was not custom-built for him, any premium would be reduced..if he simply bought it from another party, or bought it new from John's, then the premium is perhaps at a minimum. If he'd won some races on it, that'd be nice, of course.
I have no idea what the premium for Boyer-provenance would be. But there probably is one.
If it has confente's trick parts (panto'd and enamelled stuff, mostly), value is increased. If it was built up from stock OEM parts, value is a little less. If the parts on it are all-original from Confente, or even from John's, value increases some.
All by way of saying that valuing a bike like that is a tricky business, and requires a thoughtful buyer. 6-8K sounds likely, to me. Depending on various details.
Heaven knows where the guy heard a rumor about a 20K confente. Anyone here heard that one? Maybe a bike for a famous person might climb up into the stratosphere like that...but other than that? Hard to figure.