Can ECC UDIMMs run at full speed with Gear 2 on a W680 board with an i9-13900K?

Yes, you can run ECC UDIMMs at full rated speed with Gear 2 on a W680 board with an i9-13900K. The chipset doesn’t force a frequency penalty for ECC like older platforms did. The catch is the memory kit itself—most DDR5 ECC UDIMMs top out at 5600 MT/s because the ICs are binned for stability, not speed. If you buy a kit rated 5600 CL46 or similar, Gear 2 is the default for that speed anyway.