If you want to build a steam machine in MINI-ITX, this is the processor to use. Keep in mind Mini ITX is NOT Micro-ATX. My power supply is external and it's smaller than a Wii and runs HEAVEN (the most taxing) unigine bechmark in the 25fps+ range. second GPU, no ugly 400W power supply, no big space. Just a unit smaller than the Wii and has the between XBOX 360 / PS3 and XBONER / PS4. Not to mention, anything even remotely compariable on intel's side is 4x's the price.
I would be interested to hear the details on your Mini-ITX build as I am looking at doing one also. I'm also interested in your PSU that you are going to use.