When it comes to this topic, I cut my teeth in the 2-stroke world were pipe length, expansion chambers, power valves, etc can all have very significant effects on not only peak power, but power and torque over the entire operating range.  I have found that 4 strokes, (especially the large displacement, slow reving v8's that we love on this board) have a much higher tolerance for bad exhaust system design.  This is primarily due to more power being lost due to exhaust system back pressure than gained from scavenging, so reducing back pressure is typically the better way to free up more power.  The turbo guys are particularly aware of this as they can generate back pressures of 50+ psi at the piston.  But as far as tuning your pipe length to generate a scavenging effect alla 2 strokes, it is very difficult to make it happen over more than a couple hundred RPM range.  This is due to the nature of pressure pulse frequency and how the pulses merge and act together within the pipes.  This effort is probably worth your time on a race car that operates in a very narrow RPM band, but on street cars that operate from 800 to 5500+ rpms, the juice just isn't worth the squeeze.  As Jay pointed out, the best thing to do is keep your back pressure minimized, via smooth bends and correctly sized pipe for the flow requirements.