Fix your exhaust leak first, then diagnose from there

Are NGKs recommended for your car? In my experience it is always best to run what your car was designed to work with (plug type, gap etc.). I never trust the gap that comes out of the box. You say it only happens when your car is making boost, so it's a high rpm miss. Try misting your plug wires with water at night while the engine is running and see if you are getting any arcing from them, try this at different engine speeds