What I have done so far:
1. Replaced cable and housing. - No change in performance
2. Hanger is aligned correctly.
3. Chain length is fine.

With that said, help! Is there a faster way to try and figure this out without trying the shifter and/or derailleur on another bike? Is there a way I could test these on my bike? I have issues with it dropping to a higher gear (smaller cog, 8sp.). I'm a bike mechanic so I know my way around the bicycle pretty well, I've never been able to get a straight answer on this though. My money is on a worn derailleur spring.
Thanks!