when i did that on my car, i noticed there was a slight decrease in inital torque, but it felt as if the hp had increased. the car didnt take off as fast, but once it hit 3500rpm, it broke the tires loose when it normally wouldnt. sounds great though
I lost a lot of torque and gained a few HP. I've learned to just get open up one side and you don't lose as much torque and it's not unbearably loud. Be careful though because loss of backpressure could cause you to bend your valves.