There is only a little evidence current image classification models outperform humans. The 5.1% number is just the number from one grad student who went through some ImageNet images himself - e.g. a single blog post (http://karpathy.github.io/2014/09/02/what-i-learned-from-com...). There really hasn't been a concerted effort to see how humans actually do on ImageNet.

I am willing to believe that a group of humans trained on ImageNet and without time constraints would be able to outperform the state of the art neural net models.