I got these inserts in front and back for the 27.5x4" terrene cake-eaters on my borealis flume - they were the only tubeless inserts I could find for 4 inch tires.
They are great! I could immediately feel my tires acting less bouncy and more grippy - I think the "volume spacer but for tires" effect is happening even with the insert being relatively small compared to the tire.
Been running 7psi for dirt trails with these in and it feels really good, and was able to ride on snow recently at 5psi with no squirmy feeling.
It was also nice getting the work done at tree fort - the people there were helpful and gave lots of advice, and the price was super reasonable.