The weber 26 comes stock with a stainless grate. Mine is at least 4 years old, and is holding up fine with frequent very high heat searing cooks. It’s discolored and no longer shiny in the spots where I sear, but the steel has not eroded, unlike like the regular grates that come on the weber 22. I consider the 22inch grates disposable. The 26 inch grate is definitely more permamanent.
The CI grates, if you find them, will be heavy. When you move the grill, or scrub the grates, your 26 is going to shift around, and that stresses the sockets for the legs (webers weak point). I would worry the extra weight of CI will lead those sockets welds to break much earlier than they would otherwise.
Another thing to keep in mind... good searing comes from active high heat. The fact that CI retains heat doesnt matter if you have a hot fire. The best searing, in my opinion, happens with thin steel, like a wok, or carbon steel pan. You can get a great sear on CI grates, but that will be because you have a real hot fire behind it.