The best source I'm using for details in one place is this site:
They provide test results for cameras, CF and SD cards, and readers. But it is a relatively small site and lacks results for many cameras and cards. I tested my own cards in my D800E and got results that were similar to the posted results, so it is relatively easy to test your own cards, reader and camera.
The website listed does not have test results for the XT-1. They do have results for the XT-10, and for that camera the write speed is essentially the same over a wide range of UHS-II and fast UHS-I cards. As you point out, the XT-I does have the UHS-II interface. That suggests at worst equal or faster write speed in that camera, and potential for much faster download speeds with a UHS-II reader. With a UHS-I reader, your speeds will depend on how the card and card reader translate UHS-II to UHS-I. There is a good bit of variety across readers, and some are better than others with specific cards. There may be driver updates to improve performance of your Mac's SD card reader. The related issue is some SD readers actually use the USB card and would be limited to USB speeds - USB 2.0 being much slower than USB 3.0.
This whole discussion involves a series of components, and the slowest component is the one that determines speed. The Lexar 2000X UHS-II card is a very fast card, but you may not be able to use that capability. There are some cameras where the UHS-II cards are much slower than UHS-I cards - and the same goes for readers.
Right now we are in a period of transition - card, camera, reader, and computer. I have not been able to confirm any computer that has UHS-II support. Most just don't list that level of detail in their specifications.