Since the memory controller is in the CPU I'm not sure what difference the chipset would make with memory timings. I think it uses the chipset as a way to assess how good the memory trace path layout of the motherboard is. B350 boards were much worse than B450 as a whole, for instance, with X570 the best with many if not most using 6 or more layers--as much for PCIe gen 4 to all AIC sockets but also for better memory support at high clocks.
So do you know how many layers the B550m Mortar uses? Generally speaking, if you can see traces on the topside of the motherboard running from the CPU socket to the memory DIMM sockets then it's going to be no more than 4 layers. If so, I'd suggest using B450/X470. It certainly can't hurt, though, to try X570 since MSI may have spent more time optimizing memory layout even if they didn't use more layers in construction.
Last edited by buddywh; 07-26-2020 at 07:06 AM.