I'd agree you probably won't see much of a performance gain once things are loaded when comparing B450 to the X570 boards, but using a PCIe 4.0 SSD will very likely improve boot and load times.
(I'm fairly certain for most gaming workloads, the bandwidth increase will only come into play when getting closer to 4k 144Hz, which is unlikely to be pushed out by first gen PCIe 4.0 GPUs.)
(I'm fairly certain for most gaming workloads, the bandwidth increase will only come into play when getting closer to 4k 144Hz, which is unlikely to be pushed out by first gen PCIe 4.0 GPUs.)