Having been through FCC testing, I can confidently say that the MacBook's case, and really any metal, consumer computer case, will do very little to help. EM radiation will leak through any gaps you have in that metal, and leak out any cables you have plugged into the machine that aren't also caged. For example: the power cable, USB cables, display cable, etc.
Now if you'll excuse me, I'm going to crawl into a ball and cry away the FCC testing tears.
Now if you'll excuse me, I'm going to crawl into a ball and cry away the FCC testing tears.