I would think that any positive effect could be reversed by essentially giving the opposite. Want to decrease inflammation? Get a younger gut biome. Mess up and get an older gut biome and you probably end up with increased inflammation.

There is also evidence that fecal transplant recipients take on physical and mental traits of the donor.

Quoting from https://www.abc.net.au/news/2017-05-09/faecal-transplant-sid....

"There have been people who have taken on the shape of the donor, such as if the donor is either overweight or underweight they've become more like that."

"There's even been reports of some people who have never been depressed getting a transplant from someone who's had depression and ending up with their first episode of depression after that."