Hi, Jason.
My strong advice would be to do all the keying once you’ve compiled the rough edit. It’ll take you a really, really long time to render and re-render if you keep making changes. What you might try is creating the two bgs (one for wide, one for close), applying them to the appropriate clips, but then unticking the key filter in FCP, so that you’re just previewing the unkeyed footage. Then if you want to see what a cut between a wide and a tight would look like, just turn on the key filter for the two clips you want to check, and render scrub between the two. This is what we usually do, prior to agreeing the edit. Then we just leave it all churning overnight.
Also, if you download the Primatte keyer trial for FCP, you get a free cutdown version, which we often use on its own. Worth a shot.
Simon.