I've played every Civ game, and in my uneducated opinion, graphics optimization has never really been very high on their list. I find it interesting that so many people on here comment right away about wins for AMD, or fps, or, well, anything other than pure speculation and hope for the gameplay. Civ games come out and cripple decent systems for framerate every time. Mantle won't really make a difference here. If you are running a mid-range system right now, in 6 months when the game is released, you will be unable to run it above 30 fps on max settings. But, because its a turn based game, it won't really hinder gameplay, which has always been the bottom line for the Civ games. I have no doubt that Mantle will help, but it will also run DX12, meaning that Nvidia cards will have arguments for them as well.
Say what you will, but this is a game still months away from releasing and anything can happen. And before you say it will be a win for anyone, wait until release. Look at all the games in the last few years that have claimed crossfire or sli optimization, only to release with features like that being extremely buggy or unpolished, causing people on either side to turn on their fanboy brands, and jump ship. Haha, this is happening right now with Titanfall, for both crossfire and sli, and its not even AMD or Nvidia's fault, but rather Respawn who hasn't coded for multi-gpu integration. THat doesn't matter though, as people on both forums are screaming bloody murder at their GPU manufacturers.