Yes, yes, I know, very wasteful hardware on a NAS, but the CPU was a freebie and I already had the cooler and RAM.
Spec: Core i7-6700K under clocked to 3GHz, 16GB DDR4-3000 RAM, Gigabyte Z270N-WiFi mini-ITX motherboard, Aquantia 10Gbps network card, Corsair H60 liquid cooler, 4x 4TB Toshiba N300 drives, 1x 60GB Corsair Force GT SSD for the OS, CFI A7979 chassis, 300W SFX PSU.
I can not recommend the chassis, it was terrible to work with and really fiddly to connect things like the power button to the motherboard header. Don't ask me how I got the liquid cooler in there, but somehow I managed. The drives are running at around 34-35 degrees C.
Performance is pretty decent over 10Gbps Ethernet.