There is already an ultimate weapon against rogue Cyborgs and that's the Robotics console, which allows you to no-questions-asked kill any (non-Syndicate) Cyborg after two minutes, and all you need is a head ID.
Also, maybe I'm just not understanding properly, but I never got the whole "Cyborgs are extremely robust" thing. They're immune to fire and suffocation, but they have
fairly low HP. Light cyborgs are
extremely easy to destroy, and unlike humans, there's no critical health, they just
die, and destroying the head unit also means that they're completely deleted from the round. They're also vulnerable to anyone with Robotics access on their ID, who can unlock their interface and eject the brain with three clicks (ID, screwdriver, empty hand). They can also be stunned like humans.
They're also incapable of self-healing outside of the upgrade that provides it, and can't be doped up on stimulants or healing chems.
The idea behind my suggestion of limiting pick-up capability depending on arm reinforcement was that reinforced parts carry a larger health bonus but also a speed penalty. Try asking anyone how much fun it is to play a full heavy Cyborg -- they're
slow as hell.