This is kind of stating the obvious. Running OpenMediaVault 0.5.48 as a KVM guest.
CIFS transfers from Windows 8.1 to OMV over gigabit ethernet maxed out at ~30MB/s.
Despite selecting Generic Kernel > 2.6 from within Virt-Manager, KVM defaulted to the virtual Realtek 8139 NIC.
Switching to the virtio driver mostly resolved the performance issues with CIFS transfers fluctuating from 60MB/s to 110MB/s.
Not sure what the source of the throughput fluctuation is, as it will be stable at 110MB/s for 10 minutes and suddenly drop.
I recommend using iftop on the KVM host to measure performance.