I'd strongly advise AGAINST using XSlimmer or the like on Apple applications - at least not those that ship with OS X. There's the potential for a lot of grief down the road as it _could_ prevent a future software update from installing correctly.
Use of XSlimmer (or better yet,
Trimmit, which is freeware) on Third-party applications is much less risky, though - worst thing that could happen would be that you'd need to re-install the app, and you could well save some space AND improve application loading times.