There's a guy on the 2nd page of this very thread said he notices no difference between DX11 and mantle as well.
Personally I don't actually understand how people are noticing 0.01 second frame time differences when playing a first person shooter at 60+ FPS. Would people even have noticed this without charts like the ones above?
Not a chance, it's placebo effect, they're seeing what they want to see.
It's really very noticeable to most people, but a lot of people are in denial because real world testing and results often show things that are inconvenient to their prejudices.
I can give you some examples here using Nvidia's own FCAT tool which measures frame latency, variance as well as things like dropped frames. We will compare AMD's Mantle vs Nvidia's DX11 implementation in Battlefield 4. Battlefield 4 is a good title as it has been out for ages and is well optimised for both sides and it saw a nice gain from the Nvidia wonder drivers.
AMD/Nvidia DX11
Variance and spikes will represent itself as hitching or stutters during gameplay. It's very easy to see and feel these while playing because typically the game is smooth for the most part.
Looking at the above chat we can see that AMD is offering a smoother experience at 1440P under DX11. Much less fewer spikes and hitching and a tight consistency of frame times. If we look at the frame variance in MS, Nvidia are offering 7+ in variance whilst the AMD solution offers 3+. Nvidia are offering more than double the variance in their frame times using DX11, ouch! Single cards are offering 1 for the 290X and just over 1 for the 780TI. Both very good for single cards, no issue here.
Lets have a look at Mantle.
The Mantle line is completely flat. There is not one single spike, not one instance of variance, microstutter, hitching, nothing, perfection. Surely it can't be better than single card, can it? Lets look at the variance chart in MS. Again the crossfire setup on DX11 offers a ball hair over 3MS variance. Mantle however offers 0.5MS of variance! That is lower than a single card can manage under DX.
That is why the majority of us rant and rave about Mantle. It's head and shoulders (woo shampoo pun) above anything else out there.
Remember, this is using Nvidia's own tool which they created to highlight the problems AMD used to face with frame pacing, variance and dropped frames.