I owned a phase 1 1986 model years ago when i was 18. On a standard engine i replaced the exhaust with a rally system from Devil and what a transformation.
With regards to turbocharging a 325 or 328, i think you would find it very hard in getting insurance for it. Firstly most insurance companies increase the prmium for wheels and suspension. Adding a turbo, most will just decline to insure. Just my thoughts. Maybe call and ask an insurance company what the costs would be.