For a rough idea of how CPU performance compares check out Anandtech's Skylake desktop CPU review.
http://www.anandtech.com/show/9483/intel-skylake-review-6700k-6600k-ddr4-ddr3-ipc-6th-generation/18
In short, the CPU performance difference between a 4790K 4Ghz i7 in a 2014 Retina iMac and the 6700K 4Ghz CPU in a 2015 Retina iMac is generally insignificant (between 2-5% on average with a few outliers).
For GPU performance it's somewhat harder to speculate, as even though the m395x isn't that much faster than the m295x on paper, Apple and AMD may have managed to get the thermals under control this time around which could result in a nice, albeit not earth shattering performance increase.