your application is already complete and is being reviewed. i think it's a bad idea to submit more materials at this juncture. you've only been complete for a few weeks. if you send in another essay, they'll have to pull your app from the pile, match it up with your new materials, and then put it BACK in the pile...huge hassle, it's going to slow up your app, you're being paranoid, it's not worth it. save the additional essay in case you're waitlisted.
LoveButton, I just posted in another thread that the waiting game has left me on the verge of submitting additional materials to UMich. Your wise words have pulled me back from the ledge...for now. Thanks!