Think: The CPU usage meter works by looking at OS time slices If the CPU usage meter is showing "50% idle" then RAM configuration isn't the problem.
Another thing the CPU usage is showing is that there is a roughly 2:1 bias toward scheduling things on a single socket. Since the software likely does not have explicit NUMA support, most of its code and data will land on a single CPU and the scheduler will try to keep most of its threads on it to minimize socket-to-socket overhead.
Dual-channel 1600MT/s RAM is just about ideal for a quad-core i5 under most circumstances and is mostly still adequate for a 4C8T i7. For a dual-socket 6C12T Xeon, dual-channel 1333MT/s is grossly inadequate when CPU0 ends up hosting most application code and data for both sockets.
BTW, Intel's LGA2011 CPUs have memory controller instrumentation which allows the scheduler to make decisions based on memory controller load. If CPU0's memory controller is under heavy load, it makes no sense to schedule threads that rely on code and data hosted on CPU0 on other CPUs since there is no spare bandwidth to serve them.