Would think it depends on what you change to. Racnray, I think it was, replaced his stock mufflers with aftermarket and saw very little difference with one set, but a bigger improvement with the another. The following link could be of interest even though its for the 1700s, shows kinda how one change can effect your bike and other changes.
http://www.vulcanforums.com/forums/showthread.php?t=16851&highlight=dyno
http://www.vulcanforums.com/forums/showthread.php?t=16851&highlight=dyno