Ok, collected my thoughts.
First off on the src/phys system is it again FreeNAS, if so what release, same as AIO version or different. Is that phys ZFS box using also CX3 cards connected to your Ubi 10G switch? I initially wanted to say maybe check jumbo frames on phy system but with it receiving data at a good rate I steer away from that. What is dest pool 2x 5x8GBRaidz2 pool? Is that a stripped raidz2 (al la raid60) pool of 10 total 8TB disks, 5 disks in each vdev? Any ent class ZIL device behind those. Again w/out ZIL and sync set to disabled if I interpreted that right I may be chasing my tail there again. Receive buffer size maybe on AIO ZFS VM...again feels like a stretch. If you had two AIO FreeNAS ZFS systems prior sending data at a MUCH faster rate w/ nc transfer method I have to keep coming back to 'what is different on phys system/setup' but the fact that it can receive data quickly/at an acceptable speed maybe points back to your raid60 not being able to cut the mustard.
I'm flailing here hah.