I can't remember the exact mechanism, but I do also remember reading what lipsryme is saying -- that alpha testing in particular is a special case that screws with the PowerVR tiled architectures, and that it's often better to instead just blend with alpha=0.
If anyone's got a link to an explanation, I'd be interested in (re)reading it, because this seems quite unintuitive...
[edit] From memory:
-Opaque geometry uses some clever hidden-surface-removal algorithm to completely avoid overdraw.
-Alpha blending is done separately, and is much faster than regular GPUs due to only working with an on-chip tile of the frame-buffer, rather than using the frame-buffer in RAM.
-If you use discard in any opaque geometry, then the clever hidden-surface-removal algorithm is disabled for all opaque geometry.
[edit2] Ah - alpha test means that you're not certain about a depth value until after the pixel shader executes, which probably messes with HSR. Alpha blending always writes the depth value even when alpha=0, so depth sorting can still occur before the pixel shader.