The collectors and the pipe arrangement between the BBKs and the Borlas are different. This could make the sound different and could also show a marginal power gain between the two.
Keeping in mind the 3.7L's firing order of 1-4-2-5-3-6 and the pipes in a rotational arrangement (hard not to do on a V6), with the BBK's you have each banks exhaust "swirling" in two opposite rotations (clockwise for the passenger side when view from the front of the vehicle and counterclockwise for the driver side). The Borla's on the other hand have the same pulses "swirling" in the same direction (both counterclockwise when viewed from the front of the vehicle).
The BBK's have a ball socket, very short collector and the Borla's have a long, straigh collector.
What are pipe lengths? Are they within 0.5 inches of each other??
2012 Ingot Silver Mustang - 3.7L V6, Performance Package (traded)
2012 HP White Mustang GT - 5.0L, Premium, 3.73 rear end