Others in my group went to Jax prior to my arrival and enjoyed it. I know at least one had a filet and said it was good. They also said they enjoyed Emilio's filet and indicated they thought Emilio's was their best overall dining experience on the island.
It is tough to mess up a filet and I would think most restaurants would use portion controlled center cut, cryovac sealed and precut CAB, Choice or Prime meat. The filet at Frenchy Ludo was cut from the large end, not trimmed of the silverskin or other connective tissue and in no way was close to a center cut and had no seasoning or flavor.
The Mailbox has a freezer/cooler near the entry door that had boxes and boxes of filet mignon. Not sure of they were for sale, for restaurants to pick up that had ordered them or what but I know the availability of a good filet should not be a problem. I saw a cart of US processed cryovac sealed tenderloins being checked in at Carrefour the other morning.
