The mantle difference is largely due to API though. Also afaik mantle doesn't officially support FuryX because AMD has stopped Mantle development and specificaly requested reviewers not to use mantle, or at least warned that it may not work well.
I agree though that AMD does have poor driver overhead, the draw call results back that up. However, I don't see that as a sign that they will get a load of performance in the future that wont appear on other GCN cards, you findings support that.
But the Difference due to API is removing the excessive overhead of amd's driver. Then the FX is showing nearer its true performance regardless of its mantle driver being less optimised.
Now if nvidias drivers are capable of running the game with low enough overhead to not impede performance. And the driver is not being impacted by the number of draw calls in the game etc. Then the titan x should be running closer to its theoretical.
So before asynchronus shaders come into the equation. Both cards are near equal in terms of performance. Maybe the FX edging ahead. But we would need a DX12 benchmark for a real comparison.
And with future drivers in DX11, GCN can certainly get better performance. Things became far smoother with the leaked drivers and lower overhead.
But there are also Asynchronous shaders to come, which could increase performance further. Even on 7000 series GCN parts.
At the end of the Day, things are only going to improve in a dramatic way for the FX and should improve a bit for the Maxwell-2 parts etc.