Tested in 2013 Mac Pro: 64GB vs 128GB Memory from OWC
OWC loaned its 128GB and 64GB memory kits as well as a 6-core 3.5 GHz 2013 Mac Pro for MPG to delve into the performance differences.
More memory helps with huge jobs that exceed the 64GB memory configuration, but the key question is whether it hurts other tasks, since the 128GB configuration has reduced memory bandwidth.
But memory bandwidth is one of those numbers of little significance to any particular task, particularly with the large and efficient CPU caches on the 2013 Mac Pro. All that counts is real world performance.
Read the multi-page analysis of the real-world performance of the OWC 64GB and 128GB memory kits.