BACH PRESSURE... myth or fact
Back pressure is a very misunderstood entity. The assumption is that larger pipes = lower back pressure and smaller ones = more back pressure. That is a simplistic view of things... but it is within that paradigm that the idea of "back pressure for torque, free flow for top end power" emerges.
It is not accurate.
At any given RPM, what is desired is the maximum flow. You want the gases to be moving out of the cylinders, through the exhaust system, and out the tailpipe as efficiently as possible. According to Bernoulli's law, we know that as pressure increases, flow decreases, and as flow increases, pressure decreases. So we want the lowest possible back pressure at any given RPM.
If you want an engine with stump-pulling torque, with peak power at 2000 RPM, you want an exhaust system that provides the least possible back pressure at 2000 RPM. Such an exhaust system is necessarily going to be smaller in diameter than one which is tuned for maximum power at 5000 RPM on an engine of the same displacement. That smaller diameter does not mean it "has" more back pressure than a wider diameter one! Back pressure is not a static measurement of a given exhaust system like tube diameter or wall thickness. It varies with RPM and engine load, and a tube that provides the least back pressure and the most flow at one RPM won't have the least back pressure or flow at other RPMs.
So, the exhaust that gives the stump-pulling torque has LESS back pressure than the wider diameter exhaust at low RPM. In the larger exhaust at low RPM, there is flow stagnation and turbulence, and the result is that there is more resistance to flow (back pressure) than the smaller exhaust... at that RPM.
Rev the engine toward the red line and the back pressure inside the small-diameter tube will increase, and flow will be restricted. However, in the wider tube, the previously stagnant flow will increase and become more laminar, and the back pressure will decrease.
So each of the two exhausts above has the least back pressure within the RPM range for which it is designed. "Back pressure" as a single number simply does not tell which exhaust is most suitable for any given application. It's all about flow... pressure is secondary to that. If you think in terms of matching the exhaust flow to the flow potential to the rest of the components in the engine, the confusion over back pressure becomes irrelevant.
So, go for the exhaust system that has its peak flow potential in the RPM range desired. If your engine is built for torque, you would want a smaller exhaust system; a revver will want a bigger one.
Also, flow stagnation sounds like drone! Many times you will hear about someone who had too wide an exhaust on his car, and it boomed like there was no tomorrow. Put in a narrower, more appropriate exhaust for the RPM (and displacement... talking NA vehicles here, as power adders act like bigger engines in terms of exhaust flow), and it quiets down noticeably. I had that happen with my Ford Ranger when the exhaust guy inexplicably decided to custom build me a 3" cat back on my 2.9 when I had asked for the Dynomax cat back. The noise was horrendous, and the low end was gone. I went back and got what I had actually wanted, and the bottom end was back, and it was near silent-- drone free, with the same type of muffler (Super Turbo).
Read this also.
Note this bit from the link above --
"However, Wollongong (Australia) mechanic Kevin Davis has done extensive testing of varying backpressure on a number of performance engines. These range from turbocharged Subaru Legacy RS flat fours to full-house traditional pushrod V8s. In not one case has he found any improvement in any engine performance parameter with increased exhaust backpressure."
A lot of folks interpret that result to mean that larger is better. Not so. You want the smallest system you can have that will still handle the peak exhaust flows with minimal back pressure. Going larger than you need to can have detrimental effects on bottom end torque without helping you any up top. Because it can hurt system velocities and interfere with effective scavenging.