Selenium already uses it in the IEDriver:
https://code.google.com/p/selenium/source/browse/cpp/IEDriverServer/CHANGELOG (See version v2.26.0.1)
An XPath library is only used as a fall back when the browser doesn't have a native XPath implementation (I'm pretty sure IE is the only one that doesn't have a native implementation).
A native implementation will always be the fastest.