I like most ideas here, and a subtle mix of all would work wonders.
Start with having untexted autochats voicing random self-doubting thoughts of the character, describing how he feels his hand is not his hand anymore, questioning if he is even still human, while at the same time keep questioning if what he had augmented is good enough, wanting more, newer and better hardware....
Taking the presumption that everybody who is at risk of going cyber psycho would have definitely amongst their augmentations optical implants, especially ifoptical implants being definitely worth it to have access to the hud game interface and minimap, then why not screw up the mini map a bit, place the mission tracker beacon to a small motel instead of the crack den you are supposed to take out. Have your smart-gun vision classify everybody as hostile, have the faces of NPCs changed to the appearance of the bad guy you have been bounty-hunting for months or that bastard that ran off with your girlfriend. Works especially well if optical mods like in Ghost in the Shell, where you get all that kind of info shown in your vision with zoomed image of the person covering the actual view of the person.
Change his available dialogue choices to show how he becomes more and more emotionally detached or cold, but in such a smooth transition way that the player would think it is due to the actual situation and story progress, not due to empathy levels...until one day suddenly he beats up his girlfriend because she has been unfaithful and accusing him of being no man anymore but only a machine......or did she???
Give his hardware malfunctions that indicate the breakdown of his meat (the vision turns into a field of data flows and unrendered poligon grids, his cyber ears picking up a cacophony of radio signals, his cyber arm lashes out in spasms and accidentally shatters some passer-by's head.....). Make him feel that his body is not his and that he has no control......
Add the psychosis quests to that and you get the real feel....
As described earlier, bot the actual clinical psychological descriptions of amputation effects on the mind and the book description of what CyberPsycho is, it is more about being disconnection from oneself and society than having no control over one's body. Dialogue decisions and available quests will lead to the alienation and loss of friends and family and create detachment and indifference towards the world....