And all of the performance improvements are in the GPU space. A 2 or 3 year old CPU is not that big a deal but a 2 or 3 year old GPU? I believe it's an order of magnitude slower (or more).

Here is an example of GPU improvements vs. CPU (although this is an older article) http://www.anandtech.com/show/7603/mac-pro-review-late-2013/...