Mood: Must fix Superlight... Must fix Superlight...
It's not a restrictor but the way the headers are made causes a restriction at the crossover joint where the front header meets the back. All SS's have it.
The usual "cure", if you can't afford a set of Spaghetti pipes, is to cut out the squashed centre bit and weld a plate across the top and bottom with a 3/4 inch square section tube inside it. There are pictures of the mod floating around but I can't find them at the moment...
It's debatable whether or not if makes much difference if any. It doesn't usually show any gain when put on a dyno but some reckon their seat of pants dyno shows more power/responsiveness...