The guy you are quoting clearly has never heard of intake reversion. As you make the exhaust more efficient, the intake reversion gets reduced/eliminated. When that happens, the incoming air does not go "backwards" through the booster because of reversion, you will now have a leaner mixture. You can "see" this happening on an engine dyno, the lbs of fuel use goes down, A/F trends leaner and the volumetric efficiency goes up.
As far as the HP "changing" with the rate of acceleration changing on a dyno pull, well it doesn't, only what is measured changes. There is a way to get actuate numbers. I wrote on this same subject on my website, if you want to read it go to
http://www.cen-texenginedyno.com/ and read "Joe's BS" at the bottom of the page.
Joe