Many people don't consider uncovering backstory a "reward", and usually refer to Trophies or an in game reward as the only rewards. You'll unlock repressed memories, the final one being unlocked at 75% overall Sync.
You get Ezio's repressed memories when your total synchronization reach 15,30,45,60 and 75% and you get some cheats (go check the cheat section) for getting 100% in the sequences but for total synchronization you don't get anything.
So if I want the full back-story of repressed memories, I don't need to sweat over not getting 100% on ALL missions then?! I only need to average all my synch enough to get 75% right?! I was afraid I was gonna miss out something if I don't get 100% on all my missions... I've been going back and re-doing every missions I failed and it's frustrating me!! Awesome. Good to know!!