Just put my TAG III in a couple weeks ago. Don't notice any performance differences loaded, but empty the cruising boost pressure is lower. Haven't driven empty enough to do a mileage check. Bosst needle movement is definently more even/consistent. Doesn't bounce as much. Maybe comes on a little quicker.
In immediate dollars, it's not worth it, not even for $100 less. Maybe it's saving the turbo some wear and tear. Maybe empty mileage would be up. EGT's might be lower but my loads vary so much I have a hard time deciding.