I don't agree with this post. The RPW BIG Slash, two into one exhaust is VERY DEEP and LOUD on a 2016, Vegas that has torques tubes, Maximus fuel tuner, and Andrews 413 cams, and would pull the front wheel off of the pavement in first and second gear.
Those Dyno Guys posted this on their Face Book home page.
Study Victory Freedom 106 engines. The M8 Harley engine reacts to modifications the same way. Both are 4 valve cylinder heads and basically the same bore and stroke (as a 107ci).
The freedom was a single overhead cam, air-cooled V-twin with downdraft induction. Due to the restrictive nature of their their frames, they struggled to feed themselves enough air. The HD M8 has way more options for intakes.
Both engines responded the same way to exhaust mods. Rule of thumb, if its loud, the torque is likely to suffer.