You didn't read thoroughly. The data was gained experimentally by the authors. Testing of the different mufflers in the first page was done on their LeMans wagon using only one muffler. This amplified the effects of backpressure.

The test data on the X-pipe, a Dr. Gas version, seems to show that the torque curve was beefed up under the max torque point. This I infer from the improved 60-ft times.

Maximum horsepower always occurs either at or, mostly, past the torque peak. So, maximum horsepower could be the same but the x-pipe would leave the line harder. Judging an exhaust system by maximum horsepower gains may be very misleading.

R.