I've been recently in performance optimization problem that could concern everyone here : the alpha blending.
Basically, it's just about rendering overlapping semi-transparent textured objects properly, with the right depth sorting.
Actually, if we don't want pixels with alpha zero to hide what's behind them, we have to use AlphaTesting in one pass, and Blending in a second pass.
But .... unfortunately, I just found that AlphaTest was a huge hit in performance. In fact, this 2 passes trick practically cut the global FPS by a half.
I started a thread in the ShaderLab forums about it, feel free to join.
edit : changed the title to fit the overall problem