Phenolic....."a usually thermosetting resin or plastic made by condensation of a phenol with an aldehyde and used especially for molding and insulating and in coatings and adhesives — called also phenolic resin."

There are no ceramics in phenolic.

On the first Ford Tauruses, the standard cars got phenolic caliper pistons, the cop and taxi and SHO models got metal pistons. So in that case metal pistons were more high performance.

Regarding thermal linear expansion, phenolic is 6 times the expansion of steel, 4 times the expansion of stainless steel, and 3 times the expansion of aluminum. Numbers were rounded.

So phenolic expands more per degree increase than three common metals.

Go with the metal pistons.

R.