Terry,

The following is from the Dinan BMW web site, and though the technical paper is written for the BMW E46 M3, Steve Dinan's comments seem generic to me:

"I’m fairly certain that many of you have been exposed to a “bench racing legend” that would have you believe that increased backpressure will improve low rpm power and that low backpressure will increase high rpm output. Nothing could be further from the truth. An exhaust system is sized for maximum flow at wide-open-throttle and peak rpm. All exhaust systems are “oversized” for lower engine speeds (rpm), as backpressure is so insignificant that it can’t even be measured. Less back-pressure always results in more power at higher rpm, with no negative effect on lower engine speed performance. The amount of power that can be extracted from an
engine at a given rpm as a result of exhaust design is really limited by the exhaust manifold or header. After the header, less backpressure is always better."