Yes, there is a significant improvement, and it is arguably one of the most notable improvements the 2016 makes over the 2015 because of how much this improvement is (and how rapidly it came about.)
The Iris iGPU on the base 2016 13-inch is comparable (and some ways superior) to the flagship dGPU on the 2014 15-inch! That's a ridiculous advancement for such a short period! 😱