I had that same famous smell on my brand new 2007 until I replaced the Y-Pipe with a flat black model with no cat. The smell was instantly gone! So, I assumed the cat was what was causing the smell. Maybe because it was so new. Not sure if anyone with an 02 to 05 non-California model (with flat black Y-Pipe) has noticed any particular odor, but amazingly mine was gone the day I replaced my stainless Y-Pipe that came on 49-State 2006 & 2007 models and ALL years of bikes for California (like the 2004 I just bought!) with the flat black one.
If anyone has noticed the smell with a flat black Y-Pipe, I'd like to know. Makes me wonder if the cat was actually the real issue.
When I replaced my Y-Pipe, I also installed a pair of Yoshimura TRS stainless slip-ons at the same time. But I assumed the smell was from the cat and NOT the mufflers.
Mufflers have no effect on smell being generated.
It is a function of the fuel being burned and the mapping itself.
The cat can have an effect, depending upon what the unburned hydrocarbon components are that the cat is making react.
Muffler exit position has a huge effect upon whether whatever stink there is being more or less of an aggravation.
The stock layout is the worst, as the gases exit and project into a low pressure area created by the riders back, and a passengers bike is even more effective at creating the low pressure zone.
The stink just tumbles around and soaks into the clothing.
Basic aerodynamics, as in lousy, is a huge factor here.
A low mount exit helps because of what should now be obvious reasons.