I know, you did say so earlier, but prior to that, everybody else is telling me that my HDDs are the reason this is performing a bit below my hopes.
Now, when I look at the data overall:
inside the guest, there is 550GB used on the two virtual disks.
A full backup is taking up 175GiB on PBS (big texty SQL databases..)
Restoring 550GB in 19 minutes is pretty good I suppose.
Anyway I will carry on testing other filesystems etc. It still seems odd to me that the network is barely being used beyond 1.25Gbps (175GiB read in 19 minutes)