I would suggest that you update to the most recent version of TM2 before you start though (alt+click on the "Check Now" button), as the older builds don't always pick up on the config changes straight away, and it is sometimes difficult to tell if edits are incorrect or just haven't been loaded from the file yet!
I'm still parsing this.. :)
I'll probably understand after a few more reads...
Sorry - reads like a stream of consciousness, rather than a helpful suggestion! What I meant to say was that when you change a .tm_properties file and then save it, it can take a short while before TextMate notices the updated file and applies the new settings (and I've noticed that some new settings are actually only applied to new files that are opened - don't worry, windowTitle isn't one of these!). The more recent builds of TM2 have got better at checking the .tm_properties files for changes and applying them more quickly, thus making it much easier to see if modifications are having the desired effect!
Holding down the "alt" or "option" key whilst pressing "Check Now" in the "Software Update" tab of the TM2 preferences, will check for the latest "dev" build, which is 9113 at the moment.