AFAIK there's two different pipes for the 640's one with a cat and one without, luckily bought mine without the cat.
As for the silencer, it was the first thing I changed, replaced it with a slip-on from FMF. The good old butt dyno tells me it's released a few ponies, but it's definately much more responsive with the FMF pipe (also removed the spark arrestor).
If you gonna do all those mods, you must have the jetting done, preferably on a dyno.