Hey,
TexturePacker have released a new polygon packing mode which is fantastic for creating tightly packed sprite sheets and actually optimizes the sprite meshes resulting in better performance. You can read their post on the
Unity forums about it. This works for SpriteRenderers, but obviously not for UI2DSprite. I think it'd be great if NGUI could support this, it would probably require having a sprite rendered similarly to how SpriteRenderers are.
I understand this isn't a trivial task, nor am I suggesting you do it now, but just thought I'd bring your attention to it and maybe it could be something to consider for NGUI 4 or NGUI 5
