I don't have a definate answer for this but after looking at wastegates on turbos on a number of occasions I can't imagine they'd seal that well. It's just steel on steel. I'd imagine there would be some leakage past the flapper but as long as it stays closed and directs majority of the gases through the turbine on boost then it'll work good enough. Just my thoughts, not sure if I'm right or not. Benny