if your last car was a bmw that was normally aspirated, for the most part the motors are designed to have a certain amount of backpressure... so... yes.. you will get more noise but less backpressure, less torque.. and less power.... In essence.. just because it works on a american car.... doesn't mean that a less restrictive exhaust is the key for a BMW.
a simple trip to the dyno likely would have confirmed your concerns on your previous car.
I would recommend going to a dyno now and get a baseline..... and then you will know in the future if your mods are helping or hurting..
Lone Star BMW CCA Chapter president, Chief Driving Instructor - LIFETIME Member # 119366
Past: 1991 BMW E30 M3 street car ; 1988 BMW E30 M3 track car http://ofest.bmwcca.org/