. . . and the vpiuva-monster has it! Got home and noticed that I *did* have my cable running on the high side of the cable pinch bolt of the rear derailleur (based on the Sheldon Brown picture). Although my routing was a bit more subtle such that the cable didn't bend up like in the picture, but was just on the other side of the bolt and heading straight away from the housing.
It took me about 10 seconds to get the derailleur indexed properly! Nice! I never would've guessed that such a slight difference would affect shifting so much, but that just goes to prove that intuition is a poor guide to basic physics.
Thanks!