Should I bother overclocking ECC RAM on a server board?

Probably not, and even if you could, it’s not worth the risk. Server motherboards are built for stability, not speed — the memory controller on most Xeons is locked, and the BIOS won’t give you the voltage or timings options to push ECC sticks beyond their rated speed. Overclocking ECC RAM defeats its purpose of reliability, and a single bit flip could corrupt data. If you need faster RAM, just buy the rated speed from the start.