I have found two bugs related to tab completion.
1. Setting "tab_completion": false in User File Preferences does NOTHING.
You have to modify the Default ones and restart ST2.
2. Shift + Tab does not work as expected when tab_completion is really OFF.
It just inserts a regular tab nor untabs the line.
Hope it's not hard for you to fix John.
P.D: Sry about those bug-tweets :/