Man just to reiterate, don't ever put anything but superspeed on a GC. I went to Drexeline Billiards in PA last night. They have 20 GC4 tables, purchased new in 2001 or so. (I was there when they were purchased). They have played jam up all these years. About 5 years ago, 4 of the tables that were previously triple shimmed for 1 hole play had new cushions installed in an extended fashion to make the pockets tight for one hole, without using shims. (One hole is the main game in this room). The owner told me at the time he put the Diamond cushions on it, and was very proud that the tables had new Diamond cushions. It has to be the black, not the Artemis, because the cushion is not very rounded like the Artemis is. (But disclaimer I never saw these 4 tables without the cloth).
During the day when the crowd is thin and the tables are dry, these extended rails play a little faster than the other tables with the original superposed from 2001, but not crazy fast. But at night, when the crowd is bigger, the room is more humid, and the balls are actually sticky (you can see the stickiness with your eyes) they play like pin ball machines. Just as bad as a Diamond when its humid. If you move over one table to the original superspeed in the same conditions, it plays normal speed.
I can't wrap my head around why anyone likes these Diamond cushions, be it Black or Artemis. Go to any room that is slightly humid, with old cloth, and you will want to pull your hair out at how unnaturally the balls fly off the rails. And move over one table to superspeed, and all is good.
This is my opinion, and I'm no champion, but its a super strong opinion. The top players in the room say the same thing.