As far as I know, there isn't a level cap on quest XP rewards, it just starts to take a huge number of turnins to get there. If someone level 60 wants to check and doesn't mind ruining their corrupt guard faction, it should take about 6 stacks of water to get 1 pixel of blue bar movement at 60, if the numbers I am looking at are correct. Could screenshot before and after for comparison in paint or something to look for the difference.
I know someone who did almost all of 45 with gnoll fangs while factionquesting, so the general concept seems likely to be sound.
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