Nice detective work with iperf.
I think that single-stream-versus-parallel result points at the link rather than XOA, and it matches what @Pilow saw (same CR speed on a much fatter 10 Gb path).
On your multi-stream question: as far as I understand it, the transfer is a single stream per disk, which is exactly why a per-stream cap on your circuit hurts so much, though I'm honestly not certain of the internals.
The one lever I'd try is NBD with multiple connections per VDI, which you can switch on under Advanced settings (https://docs.xen-orchestra.com/xo5/incremental_backups#nbd-enabled-backups); in principle, that opens several connections for the same disk, so it might get you past a single-stream limit, though I can't promise it beats your ISP's per-stream shaping.
For a definitive answer on whether the data mover can parallelise per disk, it's probably one for @Team-XO-Backend. Either way, glad it's narrowed down to the link.
