Yeah S4 is 28nm.
It's going to be less energy efficient, unless Samsung do magic to it (their quad core Exynos A9 is doing really good in SGS3, but A15 needs more power).
I suppose the 1,7ghz A15 Exynos will perform like ~30% faster (CPU) than the popular dual core S4 1,5ghz. Because of faster GPU it should directly compete with Snapdragon S4 Pro dual cores that are coming out soon.
I think it's not going to show up in any phones anytime soon. We'll be lucky to see a phone rocking that chipset by the end of this year.