I actually have to disagree with both of you about these FM2(+) cpu's/apu not well suit for an ITX system. They're very capable in such system.
I said it's not WELL suited. I'll try to clarify. When you're talking about a budget overclocked FM2+ rig, you're going to get better results at a lower cost with a larger build. They're not trash in an ITX build, but they are less competitive with Intel - especially when combined with discrete graphics. That's more my point, and I think that line of thinking is indirectly supported by the review results. Oh and your thermal margin numbers don't mean a ton unless you run sustained whole-system loads with a discrete graphics card (which dumps more heat into your chassis) and graph your clockspeeds over time. I think you'll find you occasionally get throttled (or miss boost clocks more often if you use turbo settings) once you start pushing voltage up even if your reported margin looks OK on the surface. Your case is also as you said not the smallest. So it's hard to compare your results to Eric's for a couple of reasons, primarily integrated graphics and the chassis.
Within the confines of the selected components, I think Eric did OK. But yeah I think the 860K would only do really well in a lower price bracket entirely, a cheaper mATX/ATX build, sub-$200 graphics, Evo 212 cooler, EVGA or Corsair 80 Plus Bronze 600W, etc. The higher your total system price, the less competitive you get. That doesn't make it horrible, just not AS good.