I typed a very long reply to this yesterday.... but the internet God's saved you all from it, by it disappearing before I could post it.
Most people, when "tuning" (or modifying) a car for street use, look to get as much area "under the line" as possible. Think about a HP line on a graph, say from a dyno-run. You want to get that line as high as possible
across the entire range, not simply shoot for a high peak hp. What's being suggested here, is that for street use, a 5% increase beginning at 3000rpm and moving up from there is worth more than a 10% increase at 7000rpm. Now certainly YMMV, depending on your own driving habits, and what the streets in your neighborhood look like.
And while we're at it; I'm sure BYP understands that talking only about diameter of the primary tube is like talking about a fraction with only a numerator, haha. The length of the primary and shape of the collector are part of that equation as well. It's amazing how much tuning can really happen with a header, though those benefits are rarely realized by a simple header change alone; the header shape should be part of the entire engine design from the start. (I'm sure that's why he suggested to the OP that he should refer to the engine builder for a recommendation)
I've built several headers over the years, from very high-revving in-line 4 cylinders, to big-inch V8's, to air-cooled-twin motorcycles..... etc etc. It's more art than science, when you suck at math like I do. hahaha.
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