Hi benjammin,
While I don’t have any personal experience with the product … Tencel is a “semi synthetic” fabric that has some attractive properties (see the comments in posts #2 and #3 here). The polyester fiber padding in the mattress pad would have “some” effect on the feel of your mattress (more than just a fabric with no padding) and the semibreathable/waterproof membrane that gives it it’s waterproof properties can reduce breathability and can lead to sleeping warmer for some people but at least the moisture wicking properties of the Tencel could help to “offset” some of this to some degree.
Hopefully any of the members here that have purchased it will see your post and share their comments and experience.
Phoenix