I have successfully run the K10s in Linux on the R720s. There is a PCI error message that you can skip and you need to modify the BIOS so that the fan speeds run at max. We get good performance out of them on our seismic processing.
Unfortunately, it doesn't work in Windows Server 2008 R2 SP1.
Haven't tried the K20s in Linux yet, will do so soon. I suspect Linux will work.
Trying to install the Windows Server 2008 R2 SP1 on the K20 results in a BSOD.