I have been running Fabric RSPAN now across the VSP7200 core for about a month without any problems. Key was to set a low QoS so as not to interfere with production traffic.
Also, I split the monitor ports across two end switches (lower two in diagram), All these are 10GbE ports and set to mirror Rx and Tx.
Understandably, when mirroring 8 x 10Gb ports to a single 10Gb port, one can expect some packet loss in the mirrored traffic. Overall with this enabled in production, I did notice a small sluggish-ness in general for the VMs and applications.
I guess that the 40Gbps links between the ToR switches, and only two Backbone i-Sids, was congested - though I did not search for evidence in performance counters.
Overall, very happy with what was achieved.