The "need" to mask shots diminishes the further away you are. If the enemies are far enough away, they don't even hear your shots, they just react to misses or kills in their vicinity.
But I don't agree on there being no need to mask shots at all, as enemies will eventually come to investigate (after one or two unmasked shots) or attack and swarm you when they see you or spot you. Unmasked shots also make them look in your direction, and the time for them to see you when you expose yourself gets drastically reduced.
They are quite tenacious when they've found you, can sneak up on you or hunt you. I've had some following me halfway across the map, and they shoot pretty well, too. Two good hits from a Rifle and you're gone. Not to mention how they shoot you, force you behind cover to heal, then throw a grenade right on top of your head, and when it goes off the run and check your last position.
Playing with Authentic HUD on SE or Authentic Difficulty and the Danger Camera turned off can get quite hectic if you happen to miss a few enemies close by and don't find a spot to hide.
Of course, if you're good, you might just make it easy by yourselves. I've mostly died when going for the challenges or secondary weapon mastery stuff, as long as I keep my distance, I manage fine.
But that's kinda the point of the game, isn't it? ;)
If anything, I think it would be nice to:
- have enemies that are alerted not go back into a normal state, but stay watchful, not go back to their patrol patterns, but move towards the general direction of the shot's origin, search, and only if they find nothing should they return to a more alert kind of normal;
- have the first shot give enemies a bit more direction of where it's coming from, and have them go towards it a bit quicker if there are no follow-up shots;
- have enemies come at you together or in threes and fours more often, not the usual "Go look if he's still there, I'll watch you get killed from here".
But there has to be a balance to it, also from the gameplay aspect. Make it too tough, and the experimentation goes out the window, the reload/retry becomes the go-to action. If you mess up, you should have a chance to get away and "reset" the scene, then try again.
Least that's what I think.