Admittedly I didn't read every single post in this thread, but I recently had this exact same issue. It's a long shot based off of what you have mentioned up until this point, but my issue was that the studs that bound my heads and turbine together had loosened and backed out causing a giant leak. I would go into boost and the car felt like it was treading water. Like I said, it's a long shot, but who knows? it happened to me.