Backpressure is that wrong word here. You don't want any backpressure. Thats like sticking a potato in the muffler. Exhaust velocity is the right word. To maintain good velocity i suggest 2.5 on an NA unless u plan on doing cams, turbo and shit. Now on some cars you will not see a difference between 2.5 and 3.0 exhaust diameter. Anyway here is some more info:
The best way to make more power with the exhuast is to set it up in a way to promote exhaust velocity. Which is not nessesarily larger tubing. The tubing diameter selection is dependant on the amount of exhaust it will have to flow.
Quote:
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"As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging"
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Jay Kavanaugh, a turbosystems engineer at Garret