I have a set of Arrows that are from a 2001 M900ie on my 620 right now. Well actually they belong to Ppoitras now. I do have a set of Termis from SocalSars S4 now. Not installed yet.
I was told by the guy at Arrow that ALL the 2 valvers use the same system as far as "S" pipes and what not. I think the differance in the Part #s you are seeing from the 600 to the S4 maybe because the S4 have catalysts in them.
cats are included in cans on '03 bikes (but not all... i think the change was done mid-swing).
fitment's universal across the lowmounts. the difference between the 620 and s4 cans for example, is size. the s4's are a bit larger for more cubic volume for the exhaust to resonate before release. this is necessary to get good power out of the motor when the can's restricted. when derestricted, it really makes no difference that i can tell.