The figures you mention, were done with canister mufflers, and one old type Krumscheid pipe for comparison
I have not extensively tested the 58 with tuned pipes yet, just have started with the MTW RE2, and 40 cm! header length. One reading yet at 6650 rpm and 6.65 Horsepower. According to MTW, the DA (to stay a bit on topic) needs about 35 cm for sweet spot tuning. For 3D, the header should be longer in order to get quick spin-up, and good full rpm limiting.
In the power curves for the Zenoah 62 is clear what short and long header pipes do to the location and peakiness of power curves. The widest curve is obtained when the pipe is tuned for the range that the engine works best in: the sweet spot. For Zenoah this is 9000 rpm.