you lose low end torque due to the fact that the aftermarket exhaust system has a bigger piping than the stock piping. The bigger piping will push out more air that the engine is exhaling than stock piping, therefore taking away a little bit of torque and giving you some high end horsepower gain. Stock piping was designed for the right mixture of low end torque and horsepower, which is for everyday drivability. However, the high end gain that you get with an after market catback system is significantly more than the little torque you lose at the low end.

Example, get a narrow straw and a wide straw. Blow on both straws. The narrow one is the stock exhaust piping, and the wide one is an aftermarket system. If you blow hard on the narrow one, you will feel a lot of pressure. This is backpressure, which translates into torque. However, you will notice that not a lot of air can be pushed out with the small diameter of the straw. The wider straw on the other hand does not have so much pressure when you blow hard into it, but more air can pass thru than the small straw, which translates into horsepower.

hope it helpd.