He was probably expecting sounds like the youtube vids where ppl block off the silencer ports in the eatons or KB 2.2 videos.
Then again, loud is relative from person to person. KB 2.1 makes a good amount of noise but is not omfg obnoxious loud or anything. I'd be more worried that it was properly dialed in, the AF was spot on, power was there and there are no leaks or issues or anything than how much noise there is. Lots of very loud, very slow Mustangs out there. TBTH, the loudest ones that are still street driven are often some of the slowest.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.