476 = actual measured hub HP.
Times a drivetrain factor, let's use 10% because we do not have roller losses associated with a typical dyno.
= 523 HP
Minus 22 HP claimed for the exhaust
= 501 HP stock...underrated by ~21 HP using the above assumed drivetrain loss of 10%.
= 525 HP stock using 15% drivetrain loss, so ~45 HP underrated.