4322MB/s using Starwind RAMDisk as a network share!!!
As it turns out the Startech PCIE riser I was using in my render nodes was limiting the HCA to PCIe2 x8 which was having a disproportional effect on performance.
I had to jerry-rig the mobo out of the chassis so that I could install the 40GbE card upright temporarily for testing. Once it was running at PCIE3-x8 (confirmed using SIV) all was good.
Thanks to everyone for their help and advice.