A-ha. Interessting! Maybe this is not a storage issue after all, at least, not related to a specific storage system. I just got off the phone with vmware, but unfortunately i do not have SDK Entitlement support, which is where the VDDK library resides, so support can't help me.
As i said before, it seems like the speed is capped somewhere. When we use direct san backup, we are utilizing the VDDK system of vmware, so therefore testing speed on a disk inside a VM may give another result. https://www.vmware.com/support/developer/vddk/VDDK-1.0-Relnotes.html
It is also interessting that emachabert can push much more than the rest of us.
What else can we test / investigate?