Performance in an exhaust system is all about flow. If a pipe is too large the velocity is too low. If a pipe is too small the velocity is too high. There is a balance at which the flow is greatest. Dual 3" habitrail exhaust works excellent on a blown bigblock but it doesn't treat your mom's 4.3L V6 in her Astro so well.
Backpressure is the measure of a lack of flow. The more restriction the more power lost to pumping the exhaust out. Any more than 1.5psi at the collector is too much.
Maybe you should ask those people why they think you need backpressure? They'll probably tell you that a little birdy told them.
my blazer currently has open headers waiting for an exhaust and it runs like a turd. The motor has 10k miles on it. I was told that when you hit the exhaust stroke the valve timing overlaps so that both the exhaust and intake valve are open at the same time for a small period. What this does is use the velocity of the exiting exhaust gasses to pull more fuel/air into the cylinder. But with open exhaust you can pull some of the new unburnt air/fuel out the exhaust port causing a lean condition in the cylinder, poor fuel economy, and backfires from unburnt fuel in the exhaust pipes...and judging from the flames that exit my headers I'd be inclined to believe it. But keep in mind this is something I was told by a mechanic and I personally have no scientific data to back it up.
It all has to do with the torque peak of the engine. If you want to rev your motor past 5000 rpm open headers or OVER 3" is for you. You have to not only change your metering but also your ignition timing. Even just doing a muffler change will change your peaks. IMHO you need no more than 2.5" exhaust on a natrually asperated mildly built truck motor. You can tune it right get your mixture perfect and have your power right where you need it, low in the RPM band. /forums/images/graemlins/thumb.gif