I can't say anything about SC2, but in HoS, the Metal renderer is labeled as "beta" – and obviously for a reason: on my 2016 15" rMBP with the AMD 460, the game keeps stut-t-t-t-t-t-t-t-tering, completely independent of the settings. There are also a few visible glitches. Between the stutters, it seems to run minimally smoother than the default OpenGL 2.1 renderer, but without means to display any actual metrics, it's hard to say.
Needs more work.
At least the number of available games with official Metal support has effectively almost doubled with that…
It's labelled as such in SC2 as well.
On my 780m GTX iMac, performance is definitely smoother, though not dramatically increased. Using Metal (or the new OpenGL 4.1 renderer) does seem to fix some of the in-engine cutscene lighting oddities I've noticed since Legacy of the Void launched.