816x has two EMIFs with interleaving enabled and is capable of serving 12 GBps theoretical bandwidth when DDR3 is operated at 796 Mhz.. 816x also provides several controls to set priority of different initators like M3/c674 dsp at the DDR based on which DDR requests will be prioritized.Although it is possible that meomentary burst acces from one initiator can degrade performance of another intiator like encoder it is unlikely .
It is not clear what the issue you are seeing is. Are you seeing momentary frame drops in encoder or do you see encoder stopping permanentl. If encoder stops permanently then it is not an issue with c674 load. It is some other issue.