I was going to comment on picking out the Leader in a Horde, but it bogs down in adding and subtracting modifiers. I think this is something FFG should have considered, meaning Leader/Horde Size differences. Could be they play tested it and they knew what to do because they knew what they meant, and we're all just gawp-mouthed fools because we don't get it. Unless…you are shooting one or the other, and not both? Though a smart KT would direct their fire at both. Well, I know how I'd do it, but you'll get mum from me.
I was toying around with this one myself trying to find something that wouldn't bog down - in the end I opted for treating it as a called shot where the penalty is equal to the current magnitude of the horde. 10 points of this penalty is ignored for every size category larger the creature is.
Chaos Arch-Heretic hiding in his Magnitude (30) throng would be -30 - picking him out is going to be tough
Tyranid Warrior hiding in Magnitude (30) Termagant Swarm would be a base of -30, but ignores 20 of that due to Enormous vs Normal, so -10 overall
As it's a Called Shot, only a single shot may be taken (no full-auto called shots), but I will allow missed shots to still hit the horde if the roll would have allowed it anyway (i.e. if firing with BS50 at the aforemention Arch-heretic, he'd be hit on a 20 or less, but the horde is still large enough for the normal +30 modifier, so rolls of 80 or less would still hit the horde). The downside to targetting the leader in this case would be that you have to engage the horde using single shots rather than automatic fire.
Also, being a called shot allows Dead Eye and Sharpshooter to apply, allowing you to half the base penalty or ignore it respectively - which makes the two Talents much more tempting (they always seemed a little lacking).
This is simple, yet elegant.
The same calculations could be used to allow these Leaders to avoid Blast and Flame weapons, providing a bonus to (otherwise crappy) Dodge Tests and allowing them to duck behind their mates for cover. Let's face it, NPC Leaders without Dodge could use the boost in survivability. Sneaky gits (those with Dodge) become a legitmiate challenge.
Now, imagine how difficult it might be to target the Genestealer lurking within a frothing Horde of Genestealer Cultists as it's running pell-mell through the corridors and braying for blood! -20 for being in the Horde, -20 for Running, -20 for Hard Target…