[Unity5] Standard Shader, Android - banding and artifacts

I've tested the Unity5 Standard Shader on several Android devices.
(Nexus 10, Samsung Galaxy Tab3 10.1, Samsung Galaxy Tab2 7.0, Samsung Galaxy S4,Samsung Galaxy S3mini).

The results vary strongly in quality. On some devices the rendering is erroneous.
Any advice on how to improve quality and/or get rid of the errors?
Any similar or different experiences with standard shader on Android? Please share - Thanks!

The following Screenshots show a set of spheres using the standard shader with different settings for 'metallic' and 'smoothness'. Columns vary in smoothness left->right: 0 - 0.33 - 0.66 - 1.0. Rows vary in metallic, top->bottom: 0 - 0.5 - 1.0. The scene is lit by a single Directional Light an a skyBox; no GI used.

On the Galaxy S4 the result was quite what I had hoped for:


On the Galaxy Tab3 there is visible banding in the specular highlight for smoothness < 0.5 and metallic >= 0.5. Enabling the 32-Bit Display-Buffer didn't improve the results:


On the Galaxy S3mini additional to the banding there are noise-like artifacts in the specular highlight for smoothness < 0.5 and metallic >= 0.5:


On the Nexus 10 the results were worst. Using Open GL ES 3.0 (device-default) materials with smoothness = 1.0 rendered entirely black except the highlight:


Forcing Open GL ES 2.0 resulted in weird artifacts for smoothness < 0.5: