I don't find the statement to broad. I see it as accurate and very much so to a degree.
I guess this truth depends on the end users use of the car followed by the ears abilty to put up woth the noise. I don't consider his build particularly powerful. I'm sure he is making a bunch of it (HP & TQ) and it is a barrel of fun for sure. The engine masters exhaust tube video is an interesting one. it was a 15.7/13 TQ/HP gain on that engine. Of course, your results may vary.....
620hp/580 - TQ open header
614/569 HP/TQ - 3 inch exhaust
601/554 HP/TQ - 2-1/2 exchaust
you're just arguing to argue.. evidenced by the fact that your first sentence ends with "to a degree" ----
"2.5 is more than enough. It is more than enough on a small block motor." That is paramount to saying: 3" is the only system you should ever use on a big block. Too broad, too blanket and just doesn't take into consideration so many variables.
You're money on with everything else, but to simply say one size fits all is blasphemy.