if you didnt change the whole piping and just did the rear section, most likely the backpressure will be teh same...however, AEROSPEED is another one of those company that spends $0.00 in research and development and just copies the exterior design of established brand names. so if you end up gaining negative horsepower/torque, it's very common with these products. I would know since (due to business reasons) i sell lots of these parts as well.
BUT in your defense, it is a very nice "style" upgrade.