There is no 'natural' limit to cybernization that affects the mind's ability to function - being a complete cyborg - that is, effectively a brain and support organs in a glorified walking metal jar, has no effect other than perhaps make the inhabitant of said metal jar question his/her existence and self-identity.
Anybody got the number to report a mad scientist to the authorities?
More on topic.
My extreme annoyance with the above idiotic claim aside, while we can't in today's world get close to any of this, the EVE universe has demonstrated more than ample tech to make the cybernetic war-machine/human combo work.
- Scientists know full well how to grow entire human bodies for mass-consumption by capsuleers and others, even if this is very expensive to your average joe, and body-moding tech is reportedly available to the citizens dumb enough/willing to go through with it. Implant tech is an age-old art mastered by the Empire, Federation and I'm sure plenty more, all arguing who is better at it. This means in short that they have the medical tech to assist people with cybernetics with any biological and implant-needed help, and help integrate the human brain and any needed supporting organs with the rest of the body.
- Nano-machine tech exists in many forms, demonstrated by nano-paste used to fix ship modules on the molecular level and even active in-battle hull and armor repair nano-pumps that flood the surface of the ship with uncounted trillions of them little things, effectively re-growing and mending 'injuries' or damage to the ship as they appear. The art of out-DPS'ing an active armor tank is the art of tearing holes faster than the nano-machines can plug them, basically. Ergo, it's not to hard to assume they know how to make the little things 'grow' a machine body as a perfect replica of the human body's functions. With this, a whole artificial nerve-system can be constructed and linked to the human mind in control of this fancy piece of hard-ware and suddenly the mental command 'move the limbs' is obeyed by the non-biological appendages you want to move about.
- Capsuleers can be trained to mentally vacate their fleshy bodies and effectively become a huge warship. They are in total direct command of this vessel; when they move fingers module racks activate and do as desired, and so on. It would be so much simpler to simply train a seriously wounded/near dead/willing trooper to master the use of a cybernetic body, becoming accustomed to it's function and all the little tricks it contains.
And when you have gotten that far, there is hardly a limit to how many gadgets and weapons and such one can install or what gear one can use. Freaking imagination limits you, and mine can imagine quite allot.
Apology for the slight de-railment.