I noticed it was doing next to nothing for me, too. I've gotten about 5 achievements while imbibed, and it didn't seem to make any difference. I guess I just figured that it was part of the new game play to have drastically reduced prizes for achievements.
I've noticed Silvertongue Tinctures working perfectly. The displayed gain on the achievement or quest (i.e the screen that comes up and says "You've gotten this or that achievement or finished this quest and earned <X>" displays the expected amount, but the numbers that appear on my UI indicating gain are correctly boosted. Might this be the issue you're having?
Well, another question is, do you have it coded so that it modifies any achievement award....or did you have to add code to each achievement and quest to allow for the silvertongue buff?