I have a TAG 1 on my '02. The only thing I can feel is that the engine seems smoother. That's about as subjective an impression as you can get. Emperically, the turbo spools a little quicker, which reduces low rpm smoke. While cruising, especially towing, the average boost is a little lower, 1-2 psi, which means less fuel is being burned for the same speed. The TAG is kind of like fuel conditioners and multi-vitamins. Better with than without, but if you were missing it, you might not notice. If you want to feel power, add fuel.
Happy New Year,
Neil