Exhaust within subcowl

I was actually being sarcastic...
My truck was godawful loud when I got it, thanks to a muffler 'upgrade'. Wasn't too long before I put a proper muffler on. As a bonus I got an instant power boost. I could tell as the 3-4 clutch had never flared before...
I've seen the same thing with bikes on an actual dyno. The aftermarket tuning trick there is to remove a bunch of torque from the midrange so that the top end power, which is actually much the same, feels like it is hitting much harder. The reality is the bike is not only slower but harder to ride smoothly. A little fiddling of 'correction factors' and a dyno curve then boasts superior power.
The trick is to only use a dyno outfit to measure changes that you paid some one else to do...