I have Unitronic stage 2. I have the same sound when pressing and leting off the throttle during slow speed. I don't hear anything turbo flutter sound under heavy load, high RPM, high speed.
I have StopTech slot rotors and EBC red pads. They are great combination. I got the kit from FCP, https://www.fcpeuro.com/products/audi-vw-brake-kit-stoptech-kit-528841kt5