I went ahead immediately with the difficulty set to Hardcore. Drones are taken down with a single sniper round to the head, but I've started fighting the Prophets (if that's what they're called, kinda look like General Scourge), and they take a couple of head shots to put down. I imagine it's more difficult to do so on Insane, where they will take more damage before they die, but that's just the nature of the game I suppose.
Where as with Halo 3 on Legendary, the enemies are 'smarter' and there are much, much more of them to fight at any given time, everyone goes down with a well-placed headshot, the exception most notably being a Chieftan, who employs armor and energy shields.
So I guess I'm saying that, at least in my experience, with Gears, the hardest difficulty simply allows ememies to take more damage, and not necessarily mitigated properly or how you may be used to in other vidoes, i.e. headshots.... and many of them (ala General Raam).

- Alty