I contacted Borla about their systems, and they don't even know what the dB levels are. They just specify a 5-7% increase in HP and torque throughout the rpm band with either system.
I've yet to contact other ehaust providers, however I'll post my finding for everyone to read.
(edit)
Using a base horsepower model of 163 hp for comparison sake:
-5-7% improvement is an aditional 8-11 advertised HP for the borla systems
-the Magnaflow dyno chart displays an improvement of 13 advertised HP
dyno-
http://www.magnaflow.com/02product/dynos/15742.jpg