Yeah, that's why I stipulated "limited" testing. That "limited testing" consisted of:
1) zoning into Skyshrine
2) conning KOS to mobs
3) using faction song sans lute
4) conning dubious to mobs
5) using faction song with lute
6) conning dubious to mobs
I well might have been on the two ends of dubious, but I was still dubious. From the wiki,
Cinda's is a 610 point faction boost,
Ervaj's flute is a 220% wind mod, and
dubious faction is either -500 to -100, or -700 to -101. Assuming the wiki is correct, the flute alone should provide 732 faction points, and the largest faction spread for dubious is 599. There is no possible way I could have remained within the same faction spread if the flute worked as the wiki indicates. Now, the song value may already include an instrument mod (if so, what amount?), or the faction scale may be larger than the wiki indicates, or some other factor.