I think you have the right idea here. If it’s a single TCP stream with no firewall rules then ~ 1.2gbps sound about right since it’s limited by single core throughput. My setup with firewall and queues got about 800mbps.
Related thread: http://forum.mikrotik.com/t/ccr1009-7g-1c-1s-single-stream-tcp-performance-limit-with-queues/117121/1