For what it's worth, when I say only 5% of players play pro guitar, I go by the career leaderboards, not knowing how many of them played one song on Easy two years ago and how many still buy DLC regularly. I know not everybody is online, but the percentages are probably similar, with the leaderboards skewed towards people who buy DLC.
As for pro guitar driving sales of a song, any increase in sales is negligible and sometimes the pro guitar song is the worst seller of the pack.











