thie high pitch noise is just the turbine wheel spinning up. nothing unusual. my 180sx did it after dump pipe, and my rx7 as well, as well as my mates 200sx and my gtr.
leakin exhaust gasket doesnt do anything for back pressure.. the turbine does most of the back pressure (80%+)
so if u have a leak after it, then it doesnt matter.. hence y race cars run just a massive straight pipe from the back of the turbo to the side of the car..