Can I use XMP with ECC UDIMMs on my ASUS X99-E WS?

Probably not — XMP on ECC memory is hit or miss on that board. ECC DIMMs are validated for stability at JEDEC speeds, not overclocked profiles, and the board’s memory controller treats them conservatively. You might get 3000 MHz to boot, but expect instability or failure to POST with XMP enabled. You’ll almost certainly have to set voltage, frequency, and timings manually.