Ok, this is a carry over from the Sisters vs. Necron's thread in Tactics. The Callidus Assassin (from Codecies Daemon Hunters and Witch Hunters) has a weapon called a Neural Shredder which does a template attack with St X AP 1. In its rules it states it is Strength 8, but considers the target's Ld for purposes of wounding, giving the example that a Ld 9 model is wounded on 5+. Now, it doesn't specify anything for Instant Death, which is figured after saves are made.
So, what's your call? Can the Neural Shredder Instant Kill a Marine, or indeed anything, ever? It seems to me that the weapon is St 8 but with a special rule on how it is applied to wound. Since Instant Death is not a part of the phase where one determines wounds (rather a part of what you do after applying wounds after saves are made) it seems to me that it would Insta Kill T4 and lower, but I can see strong arguments the other way as well (particularly from Skyth.)
So, dispense some wisdom, if you will, in the name of the Emperor.