Hi color,
They certainly have high quality quilted covers that use natural materials (cotton and wool) but I would keep in mind that there are many other manufacturers that also have covers that use natural materials as well.
You will find that some people do better in terms of PPP with the Berkeley Ergonomics tension adjustable slatted base and some that don’t (or some that don’t notice a difference either way) depending on the specific mattress, your sleeping position, your body type, and how sensitive you are to changes in your sleeping system. It can certainly help with “fine tuning” for some people but there really isn’t a formula that you can use to decide whether it would be an improvement for you or not so without trying it in person under the mattress you choose, it will really be a matter of “best judgement” based on your specifics of your conversation with them.
You can see more of my thoughts about tension adjustable slatted foundations in post #13 here and post #2 here and post #2 here.
Phoenix