Does DDR5 memory make a big difference for DirectStorage games?

No, DDR5 helps a little but not as much as you would think. DirectStorage is designed to let the GPU do most of the work, so the bottleneck shifts to GPU decompression speed and VRAM bandwidth before DDR5 speed matters. The difference between DDR4 and DDR5 in DirectStorage games is only a handful of frames at best. You’ll see bigger gains from a faster GPU with more VRAM and a good NVMe drive.