For Ravencoin and Bitcore, the DDR4 is not used for hashing at all, but rather to store hundreds of different FPGA configuration bitstreams, which allows the entire FPGA to rapidly reprogram itself on every block based on the algorithm sequence for that block.  Cryptonight7 and Equihash, on the other hand, would use the external memory for actual hashing.  Whether CN7/Equihash gain from additional RAM depends on how those algorithms are ultimately implemented.