I have the metal set on my CV and it's worked fine for me, but you really have to watch alignment as "thepinballworks" noted above. You have to ensure there's no way that the Ringmaster is going to catch on the metal playfield protector ring when he raises.
Before I installed the metal protector, I got some cool ball action from the Ringmaster playfield hole itself. If I shot the ball at the hole and hit the extreme left or right of the hole, the ball would start to go into the hole, but the g-forces would whip the ball around the edge of the hole, like an inner loop shot, and the ball would come flying out and back at the flippers, which was pretty sweet and cool looking. I was worried that the metal protector would change this behavior, but I found it didn't. I like seeing the ball arc around the Ringmaster hole like that on a glancing shot. I wonder if the plastic protector changes the way the ball whips around the hole on a glancing like that? It looked to me like the plastic protector may stick up farther and it may have a little lip on it which might make the ball "jump" a little. Can anyone with a plastic protector comment on this? I've always been curious.