For my insanity run, I used:
Assassin Cloak (Cloak 4 with damage boost)
Heavy Disrupter Ammo
Heavy Incinerate
AI Hacking 1
Tungsten Ammo (AP 4 heavy)
On reflection, extended cloak might have been more useful, since I found it to be way more effective as a way to find new cover than a damage bonus (throwing an incinerate immediately after the shot was roughly comparable if not better). The disrupter ammo was handy, but not crucial.
There seemed to be no real replacement for AP rounds. The most difficult encounters in the game for me were inevitably encounters with Scions; in normal I could essentially one-shot them, but in Insanity it seemed to take a minimum of 3 shots (with full AP, headshot buffs, widow and even cloak bonus - without AP it was just ridiculous). I tried basically every other alternative power (except Domination, which I didn't have), but the sheer killing power of the AP Widow is hard to match.
AI Hacking 1 seemed like a very good investment. Yes, after you drop their shields/armor, you can kill them quickly - but if you hack them, they'll typically be able to get the shield or armor off the next target, which you can then control (killing the original, nearly dead one quickly). You can 'chain' your way through a group of enemies this way, and even Hacking 1 is enough to permanently control a single basic enemy. Also works surprisingly well with heavy mechs - blow through shields with Overload (from friends) and SMG, knock out armor with Widow, then Hack it for serious help. The explosion also does a great deal of damage when it dies.