Hello please give this report a good read as I have seen this problem in other games and it keeps happening.
There is a problem with the aiming acceleration that makes any rapid and accurate adjustments to point of aim almost impossible. I suspect this is why you had requests to add an aim assist in game.
The acceleration on the Y axis is not proportionate to the X axis. This is a difficult problem to describe but I have tried and no amount of adjustments to the X/Y settings in the menu will fix this. The problem is rooted deeper than that. It is extremely difficult to tell exactly what the problem is but what I can say is this.
You MUST make sure that the thumbstick movements respond in game EXACTLY the same between movements along the X axis vs the Y axis. (assuming the X/Y) settings in the menu are equal)
To put it simply it feels as if at the moment on the Y axis always begins moving sooner than it does on the X axis regardless of sensitivity.
This makes it impossible to become familiar with the aim and make predictable rapid movements (twitch shooting).
To give you a very obvious example.
"FarCry 2" on the xbox 360 had this problem but waaaay worse. Play that game for a while and see what im talking about. If feels so much harder than it needs to be.
Now play "Call of Duty Modern Warfare 2" on the 360. I maintain that the aim in this game was incredibly intuitive even when just looking around the environment where aim assist had nothing to do with it.

I hope you took this seriously and will have a good look into this and if theres anything you would like me to clarify just let me know.
Thank you