Those hardware texture compression formats are lossy, so you’re going to notice some banding. I suspect that that is unavoidable. However, we should be deploying a couple of things very soon:
ETC2 support (we’re adding ETC2 as an additional option in the texture compression panel, which will allow you to compress textures with alpha on Android).
Compression quality setting to enable you to maximize the quality of your compression (if you’re prepared to wait for the additional compression time!).
1 should be done today. 2 will be done in the next week or so.