According to the marketing material the Iris Pro 6200 can be as much as 2x faster than the HD4600 found in Core i7 4790S. Now we all know that much of the claims in marketing material is best case performance and need to be taken with a grain of salt, but even given that the performance improvements that you can expect in gaming and compute should be noticeable.
However, there is one item outside the realm of 3D gaming that pushing Iris Pro will bring Intel: the content creation market. This market has recently been dominated by NVIDIA with their close ties to Adobe and other content creation developers. Especially in the mobile market systems without serious support for OpenCL fall by the side when it comes to content creation. I know of several people that have attempted to get by with Intel HD Graphics only to spend more money to pick up a laptop that has full OpenCL support. This is one of the reasons that the Yoga 2 (and 3) Pro were not more of a hit and why the Surface Pro still cannot pick up the real content creators.
By dropping in Iris Pro this will give Intel an advantage over both NVIDIA and AMD. They can offer Adobe (and others) supported GPUs in a smaller and more power efficient package. OEMs can now offer multiple levels of performance to their customers without the need to spend extra on NVIDIA or AMD discrete GPUs. Gamers will still want those more powerful GPUs in their systems, but the average editor will be able to make good headway running Iris Pro and save themselves some money in the process.
The new line Broadwell line up should make some very interesting waves in the market. We hope to get one in the lab soon and put them to the test running games and, of course, some of the more stressful content creation apps that we can throw at them.