Not related to your question per se, but note that Metal on iOS might not be the panacea that Apple makes it sound to be. In my experience there are huge efficiency problems with it.
I have a game made with SpriteKit (with a bit of SceneKit thrown in as well), and it's very easy to switch between using OpenGL or Metal with it. On an iPad Mini 2, I'm getting a very stable 60 FPS framerate when using OpenGL (there's no easy way of measuring how much higher it would go with an uncapped framerate). However, if I switch to Metal, I'm getting a relatively stable... 45 FPS. And it obviously looks horrible (movement is very jittery because of all the lag frames).
I do not know if this problem is in Metal itself, SpriteKit, SceneKit, or something else, but it makes Metal unusable in my case. (The most worrying thing about all this is that Apple is deprecating OpenGL...)