Had to fix part of your post.
You cannot fine tune a SP campaign for years and expect newly patched in controls to not drastically alter the intended difficulty. If you suddenly made it twitch aiming it would be too easy, the AI assumes a certain amount of time to aim, reload, move, etc.
Maybe we're playing different games, because that's not my experience at all.
The AI has always reacted extremely quickly to my actions, be it a Grenade, or firing at them. They'll dive for cover if need be, or remain in cover if I fire shots over their head. If I advance too far forward on my own, they'll rush me instead of holding back in a fire fight.
It has not been my experience
at all that the AI is "compensating" for the way you aim, with delayed reaction to move, reload, shoot, etc. No compensation at all.
This whole "breaking the game" theory has been taken too far. In certain instances, I can understand it. 2 player split-screen co-op, tracking, that's tough to do without changing the game. Changing weapon balance and reload speeds? Got it, that's going to change the game considerably, but it won't break it.
Changing the response of aim input, or accelleration...I don't think we're looking at anything game breaking here.