Steve and Jim spent dozens of hours on the dyno with their pipe designs. Trying numerous permutations of length, diameter, bend radius, megaphone shape, etc, etc, etc, it’s such permutations I’m referring to, why weld in 4”? Why not 2,3,5,6,7 etc, etc?
So yes, when they say something works, it’s usually very well proven. So yes, I’d tend to believe them.
I put a Maney pipe one mine and it gave a 5 BHP difference, even more in the mid range. After having the head ported and the followers radiused to reduce duration, all of those gains disappeared, there was virtually no difference between it and stock pipes and peashooters anymore.
What this taught me was that a surprisingly small difference in cam operation can have a huge impact on exhaust function. A simplified rule of thumb is, the more radical the cam, the more sensitive to exhaust tuning.
So, as Chris‘s pipe is of unknown make, and certainly not the same as your one off, and he is using a radical race cam, etc. I basically stand by my point that modifying it would need to be done carefully.