In my experience.
You must instal de Leo with the cat-pipe, cause the Leo pipe's are wider then the original exhaust pipe's.
The easy mod is to fit a 07 Leo to an older FZ6, cause then you just need to shorten the pipe. Wich is quite doable @ home.
The other way around, older Leo to 07 FZ6 is harder cause you wil need to lengten the cat-pipe. You will need tigg welding in that case.
Basically the only difference on the Leo's are the cat-pipes, but you can't fit the Leo can's on your stock cat. Cause the pipe from the cans and from the stock cat don't match.
I have a 07 Leo on my '04 ... they fit. Make great extra power (98.1hp @ the wheels with CO tuno - no PCIII) and sound awsome. However, the fit wasn't 100% flush and it feels a bit "crooked" at the back.
If you want fitting pipes and no hassle, get the ones for the '07 models.
Peace