If you add an extra muffler to the occarino type side dump muffler, a lot of power is lost. That side dump muffler is not great to begin with, because it is way too small, and this discrepancy is reflected downstream if you add extra restrictions.
OTOH, If you add the same type of aftermuffler to their black tuned muffler, all the gains are yours for the taking, both noisewise and powerwise. The tuned section only sees the increased back pressure, which in this case is beneficial, because in these engines it aids the scavenging process efficiency. (beware, this is not a general rule!!!)