But...while we're nuggin' out the details. An NA car doesn't require "back pressure" as you put it. Common misconception. So to help make vortexers smarter than the average bear...
...the true tuning element in an NA exhaust setup is exhaust flow and velocity where the engineering target is to keep the spent gasses moving outward rather than allowing them to slow, stop, or reverse direction even momentarily. Higher velocity exhaust movement is more resistant to such unwanted flow reversal and can even help pull the next burst of spent gases along. This is the design principal behind your typical header. On an NA engine, tuning the exhaust system usually means having a target rpm in mind where peak efficiency is desired. In small displacement engines, the target rpm range is usually higher where the engine needs to spin to make it's best power output but low-rpm power usually suffers. On large displacement engines, the torque band is so large that finding the sweet spot for exhaust tuning is much easier and tends to be biased lower in the rpm range. But anyway, exhaust gas velocity is higher with smaller diameter tubing. This works great in the lower rpm range where the gases are better scavenged by the collectors but as rpm rises and exhaust gas volume increases, the smaller diameter pipe becomes a restriction to flow and "top-end" performance suffers. Opening up the pipe to allow for better flow at high rpm reduces the velocity of the gases at lower rpm and can allow the unwanted stoppage or reversal of gas flow making it harder for the next exhaust pulse to exit the combustion chamber and certainly not helping it out. This can cause the car to run like poo in the rpm range where most of the 'daily' driving is done. This association between exhaust pipe size and engine behaviour leads many to think, in the simplest terms, that the engine is performing better due to the increase in back pressure.
Break for recess.