How efficient is fragment discarding for large amounts of triangle overlap?

I have a question regarding the efficiency of discarding fragments for scenes that are excellently sorted, but have many overlapping triangles.

If you have a large number of large triangles (eg. 1000) that are perfectly front-to-back sorted but overlap (or occlude) one another, roughly how much hardware performance penalty will this incur? I understand that the sorting will result in zero overdraw and thus fewer fragment's being processed by the fragment shader, but in this case will Mali be able to discard fragments that are not seen at a very low performance cost?

If there is a significant performance cost, what is a rule-of-thumb maximum amount of triangle overlap before performance starts to degrade?

Sean