Eh, you ain't missing much but yeah, it'd actually make RE:CoM a little more worth it but to tell you the truth, FM+ was only worth it in itself because of RE:CoM in the first place, LMAO.
As a person who actually has experience in owning FM+ and finishing it all n' shit, I'd say buy it if you never got FM+ considering FM+ isn't the main treat based on the packaged game duo and it's overrated, the new boss battles ain't very fun you know, just new challenges that leveling to 99 won't do you good at all and it's all based on skills in critical mode is all, woohoo.
Still, I personally enjoyed the challenge of critical mode, although the difficulty AI level itself doesn't increase at all, it's only hard as shit because your life stays so low instead of going high, that Terra gave me a run for my money, the Roxas fights'(both story's and data rematch) are easy as hell though. Still, all of that missing from a player's experience shouldn't shoot down the fact that they could still enjoy the main sweets of the FM+ which is of course, RE:COM. >_>
Overall, If a person already owns it already, then don't give a fuck at all, they should just go on the net and watch the scenes, it's not worth it to play again. *raises finger to the last Fake Riku fights in RE:COM* So obnoxious, those fights!