I do have a bulge before the Y (TWSS). If I have a cat is there any reason not to switch the Y to an earlier cat. I have a feeling I'll be using the search engine tonight to do more research.
1. The earlier pipe is not available any more from Honda, who will only sell the next to last catted pipe (part 18380-MCZ-D12) for earlier models. That one came from the 04-05 California model.
2. 06-07 bikes have a slightly larger cat due to the 06-on standards. That one's 18380-MCZ-D13, no matter whether it was CA or not.
Here's the part number breakdown:
18380-MCZ-611 - 2002 to 2003 California emissions pipe
18380-MCZ-003 - 2002 to 2005 Federal emissions pipe
18380-MCZ-D12 - 2004 to 2005 California emissions pipe
18380-MCZ-D13 - 2006 to 2007 50-state emissions pipe
D12 and D13 are the only ones that Honda will sell any more. D12 is what the pre-06 bikes are supposed to use and D13 is what the post-06 bikes came with and are legally obligated to use. Interesting to note that there isn't a -000 version.
I have *heard* that the 02-05 pipe could sometimes be had with a bulge that had nothing in it, i.e., was catless from the factory. However, that does not match what I've seen - every single US market 919 'bulge' pipe that I've looked at has had a cat in the bulge. Euro models were different, of course, and they had no-bulge Y-pipes for some years, I don't know what the breakdown is for them.
I have a relatively early production bike (MY2002, 1500 series serial number); the pipe appears to be original and when I took the pipe off to check it several years ago there was a cat in the bulge. This is a 49-state bike that was originally sold right here in the Dallas area. I'm the third owner; the second didn't replace it (actually he didn't replace *anything* even though there were things he should have) and I can't get ahold of the first one. The dealership he bought it from and had it serviced at didn't have any records of the pipe being replaced.
Reasons not to switch: Exhaust stench will get a *lot* worse and you'll have more soot.