They really are! It seems to me like sidehack did good work on the thermal design. Super beefy heatsinks on both sides, with nice channel for the air, all in all running great.
I've ran 600MHz for over 48h without touching the voltage knob, and no anomalies in error rate or anything.

Now testing 625MHz just using --gekko-r909-freq 600 on all cores.

Nice setup! For what it's worth, you could run multiple cgminer instances all on a single Pi, or are you getting issues when too many USB devices are connected?