For my E30 325i, the biggest performance I gained from cleaning the injectors. When I bought the car I drove it 2000 miles around Europe and noticed sluggish drive in speeds under 100km/h.
Pulled the injectors out and it had what looked like decade full of gunk. Spray pattern non existing. Just weeped fuel through the gunk. Gave them ultrasonic bath and rebuilt them. Car now feels very responsive in city driving and accelerates nicely in highways.