Status update. You can completely disregard my earlier findings. The issue was with the test client I was using on the VLAN tagged side. When adding a VLAN tag the NIC didn’t automatically adjust the MTU correctly so was discarding input frames. 
Manually forcing a higher MTU to account for the additional VLAN overhead resolved the issue and I cant get the same speed results I was seeing before lowering the MTU on the Beryl.
Back to square one and can’t reproduce your issues on my unit so something else is at play in your case 