when AMD launched the Fury X, it told the press that moving to HBM reduced power consumption by 40-50W. Thats power that can be redirected to the GPU and spent on increasing fill rate and texturing (to take advantage of the increased RAM bandwidth).
At 256GB/s of bandwidth and 8GB of RAM per die stack, a single HBM2 link at half-clock speed would still give an AMD APU 128GB/s of memory bandwidth, or roughly 4x the realistic peak you can hit today. At that point, the distinction between current midrange cards and an APU becomes irrelevant (or more appropriately, governed by questions of wattage, mobility, and heat, rather than intrinsic limitations of the platform).
AMD hasnt announced any HBM2 APUs at this point, but has stated it plans to extend the technology to all facets of its products, which heavily implies that well see Zen APUs with HBM2 at some point in the future.