In my experience, you can frequently get away with using a few keying filters on a clip (be it in FCP, AE, Shake…whatever), but it’s not necessarily the BEST key, but may be passable. Complicated keys could require you to slice your layer down the middle and key each side of your frame separately. You may even need to animate masks where a key is not working correctly… these are not tasks I’d like to take on in FCP.
To answer your question though, Shake has two very good keyers included, offers real time composite preview frame by frame of a key (not RT palyback – you need to render a preview for this). You also have a great amount of control and freedom in Shake, and yes, it renders faster.
-Ben