Well all i know is it becomes less scary for me.
But that kind of stuff is always very subjective, I mean there are even people out there who thought Amnesia wasn't the least bit scary but who were scared as hell by Doom 3. o.O
Good that you mention Manhunt though, it definitely needs to be added to the list of disturbing games. Although I think one could almost call it a reverse horror game because the things your character does to survive are (sometimes) even worse than what your enemies will do to you if they catch you. I remember Yahtzee once said in a review it felt like being in a jason film with the roles reversed - where the only normal dude hunts down lots of deranged killers
Buuuuuut - even Manhunt suffered from turning into a shooter halfway through. And that's the point where I stopped playing and played Manhunt 2 instead.