On graphics without Mantle (Mantle delayed until next month)
On Mantle potential:
http://hardocp.com/article/2014/02/26/thief_video_card_performance_preview/3#.Uw4MBmd3uUk

In our first graph, we are comparing the Radeon R9 290X and the GeForce GTX 780 Ti. There seems to be a big difference in performance between the two, with the GTX 780 Ti delivering better performance by 20%. However, both are insanely fast averaging near 100FPS and performance well above 60FPS at all times. Remember, this is the highest settings possible in the game, all graphics options turned on.
On Mantle potential:
BF4 is a good case scenario for Mantle. However, a game like Thief isn't like BF4. Thief, being a singleplayer game, is more about the experience, and less about needing the highest possible FPS for competitive gaming. Given the fact it already produces performance that is more than acceptable, what more can Mantle bring to the table to improve upon the experience?
http://hardocp.com/article/2014/02/26/thief_video_card_performance_preview/3#.Uw4MBmd3uUk