Interesting info in these two sites

 http://www.rbracing-rsr.com/exhausttech.htm

http://www.harley-performance.com/harley-exhaust.html

Unfortunately, your Harley dealer will not tell you any of this because they are just too happy to sell you a few sets of slip-ons and for you to lose money every time you sell them on second hand.

Another thing, I think Harley Davidson has decided to locate the cat converter in the head pipes for a "good" reason; to satisfy government authorities because most Harley owners leave the stock headers on and just change the mufflers for a better sound. This way the emissions are taken care of egardless of whether you keep the stock exhaust or change mufflers. In California it's illegal (you can get big fines) if you remove the cat converter from your vehicle. In addition to this Harley reduces cost by installing one cat rather tan one in each mufler.