We have r730's with E5-2699 v4, BIOS 2.8.0. Ours are suffering from cascade failures. First the servers crash and we got messages about the CPU failing in the lifecycle log. Then everything (HBA,NIC,etc) reported a failure of some kind, then the drac would become inaccessible. We got up to Dell engineering before they gave us a response to disable QPI Link L1 Power management. Apparently the setting has a microcode issue they did not disclose, and then set it to enabled by default in the 2.8.0 BIOS. They instructed us to disable that feature.
System Settings > BIOS settings > Processor Settings > QPI Link L1 Power management
Whether or not this works is still up in the air. I'll be disabling it on roughly 70 servers. heavy sigh