I see we went full circle with the amount of DIMMs necessary to fill 16 memory channels 
Single-rank (more precisely: one rank per channel) can generally be overclocked to higher frequencies. Something that does not apply here. At the same frequency, dual-rank delivers slightly higher bandwidth.
Using more than one or two ranks per channel can cause memory frequency to be dropped lower automatically by the motherboard, especially on older platforms. Maybe that's where the confusion comes from.
Single-rank (more precisely: one rank per channel) can generally be overclocked to higher frequencies. Something that does not apply here. At the same frequency, dual-rank delivers slightly higher bandwidth.
Using more than one or two ranks per channel can cause memory frequency to be dropped lower automatically by the motherboard, especially on older platforms. Maybe that's where the confusion comes from.