The reason Zen1 (and 2) speed up a lot from ram speed increases is because the memory controller speed is tied to ram speed. So when you bump up the ram speed you also bump the memory controller speed, which reduces ram latency and inter-cpu communication latency.
There is no telling what the memory controller -> ram ratio will be with ddr5, the memory controller has speed limits, so you aren't going to get free speed just because the dr5 starts at 4800, because the zen memory controller can't run that fast anyway.
