40k - Ork Mob Rule vs Psychic Shriek
In the Letter of the Law:
Psychic Shriek asks you to evaluate the target unit's Leadership value. It does not say that it causes them to make a leadership test, so it can still apply to things that ignore/auto-pass leadership tests.  

Ork Mob Rule says you (meaning the Ork player) may always choose to substitute the number of Orks in the mob for their Leadership value. It does not have a limitation of when this can be done (so technically, randomly in the middle of the shooting phase, you can just declare "oh, and because there's 9 of them left, their leadership is 9").

In the Spirit of the Law:
Orks are either so dumb they don't know they should be afraid of things even if they out number them, or so incredibly brilliant and powerful that we mere 'umies are unable to comprehend their courage. This invigoration seems to come from a quick head-count of their numbers and the raw logic that US > THEM == VICTORY. So when attempting to scare the spores out of a group of Orks, you have to consider that either they get a little scared, but after looking around for help find themselves in sufficient quantity to handle the threat, or they realize they are confident in the prowess and abilities of their compatriots and simply dismiss the threat.