The cycle ergometer was invented in the late 1880s, and VO2max was first measured/recognized starting in the 1920s. IOW, there are plenty of data from before the widespread use of blood doping/the invention of EPO. For example, Merckx reportedly averaged 6.2 W/kg during a 1 h ergometer test in 1975: http://forum.cyclingnews.com/showthread.php?t=8170
Actually, I was neither a co-author nor a participant, as that study was performed after I'd graduated.
I don't believe that any of the subjects in that study raced on a European pro team, much less, e.g., challenged for the GC in one of the Grand Tours. Thus, I don't see the relevance of the data. (OTOH, most would hold up Graham Obree as an example of a clean rider, and his FTP was ~6.1 W/kg.)