Is DDR5-8000 CL38 stable? Bleeding-edge Intel XMP and timings guide

How this is calculated

4000 MHz bus clock, 0.25 ns per cycle, 38 cycles of CL = 9.50 ns. Peak per-channel bandwidth is 64 GB/s, 11% over DDR5-7200 and 25% over DDR5-6400. Four-DIMM configurations almost always require backing off to DDR5-6400 or slower on this same platform, which is why enthusiast builds targeting 8000 MT/s universally use 2× 24 GB or 2× 48 GB kits rather than 4× smaller modules.

Verdict

At 4000 MHz bus clock the 0.25 ns cycle and 38 cycles of CL put first-word latency at 9.50 ns, essentially matched to DDR5-7200 CL34, and peak per-channel bandwidth hits 64 GB/s, 11% over DDR5-7200 and 25% over DDR5-6400. The constraint that defines the use case is framework: four-DIMM configurations almost always back off to DDR5-6400 or slower on this same platform, which is why builds targeting 8000 universally use 2x 24 GB or 2x 48 GB kits rather than 4x smaller modules. A high-end Z790 or Z890 board with a strong-IMC CPU and tuned case airflow over the memory slots is the realistic kit, and the gaming gains over DDR5-6400 are modest relative to the price premium, so it earns its keep on bandwidth-bound synthetic workloads rather than gaming.