It has been confirmed to me that the ER-X / ER-X-SFP have only one 1 Gb/s link between the SoC and switch. Since every packet that enters the SoC through that link will have to exit the same path, 1 Gb/s is the maximum aggregate throughput.

I’ve not been able to garner any interest in why bi-directional testing take a substantial performance hit. I may try some earlier firmware releases in the future but for now I’m moving on from this subject.