A reduction of backpressure will not by itself create a loss of torque in the lower RPM band. First, most people only think they have lost torque and never actually do an apples to apples before/after dyno comparison (along with A/F ratio readings). If there is a loss of torque it will directly be attributed to a significant change to the A/F ratio. If you add headers the fuel trims will begin to go positive. This means that the factory calibration is not correct for the new configuration and the computer is having to add a compensating factor to the injector pulse widths. This is a good thing but it also can have a negative effect at wide open throttle where the factory already tends to delivery a richer A/F ratio than is ideal from a performance point of view. Being too rich will cause a loss of power.
I've been tuning (PCM calibrations) and modifying GM LS1 based vehicles for 6 years and have never seen a loss of torque result from a reduction in backpressure. Once the A/F ratio is brought back inline you will always make more power if there was a restriction in the first place. Another common thing is that because the exhaust note has become greater in amplitude the feel doesn't necessarily equal the new sound. Have you ever seen the guy at the track that sounds like top fuel but only runs 13s?
Tim