The proof of concept demoed at MWC 2018 delivers sequential reads of up to 880 MB/s and sequential writes of 430 MB/s, which are significantly better than what the current UHS-III standard ofers. However, WD says that PCIe SD cards will not see commercial use any time soon.
https://www.notebookcheck.net/Western-Digital-shows-fast-SD-cards-with-PCIe-X1-interface.286890.0.html