There are a whole lotta factors in tuning exhaust. One aspect of it is sonic, and depends strictly on the length of the header pipes regardless of how well the ports or rest of the system flows. The sonic stuff is something the Volvo dual-downpipe system does well -- even with the same stock ports and small exhaust tubing with multiple mufflers, power went up about 6% from that change alone. It's not just that the two-pipe section is slightly less restrictive than the older (and larger) single pipe. It's actually tuned.