What kambites said (which seems to often be the case when it comes to powertrains!). It's a fairly common conception that back pressure is bad, with the idea being that it stops exhaust flowing effectively out of the cylinder and robs you of cylinder space for the fresh charge. However, it's not as simple as that, and while having no back pressure is better than having loads right by your cylinders' exhaust valves, what you actually want to do is shape the manifold (and engine timing) to cause some pressure elsewhere, and a vacuum near the valves. As there's only going to be a finite amount of gas in the exhaust, deliberately raising the pressure in one place reduces it in others, which is beneficial when done correctly.
If you make the exhaust flow more freely, yes you get less obstruction to the exhaust gas escaping, but you also lose the lower-pressure vacuum near the cylinders, and don't scavenge all the exhaust gas out as effectively. Resonance waves exist in the manifold (this applies to the intake one too), and are very finely tuned so that each cylinder's exhaust gas assists the next one in scavenging more thoroughly. Buggering around with it makes you unlikely to achieve the same benefit.
On a turbocharged car, as has been said, you cannot scavenge effectively anyway so needn't worry. But for naturally aspirated engines, while "too much" or "incorrectly placed" back pressure is bad, do not think removing it completely is the best way forward!
raise: loving it. Awesome sound. Now for a mid pipe. :thumbs::thumbs:
