I did this test last night with my buddy's 6K RED RAW footage. I could play 6K realtime at 1/4 quality, while my fully loaded MBP with i7 + 32GB of ram could only playback at 1/8 quality. Keep in mind real editors NEVER edit raw footage, they use proxies.

The really impressive part was that the mac mini did NOT spin up the fan during playback, completely silent. The i7 Macbook Pro sounded like a jet turbine spinning up within 30 seconds. Awesome.