I could see the proposed system working as a possible method for determining how much space a body should have, while the actual space required for a bionic could be determined by looking at existing technology and items. However, I have to wonder if a “space” system is moving in the wrong direction, and perhaps we should consider another method for limiting bionics or making them more realistic.
The first one that comes to mind is integrating the encumbrance system. I.E: Each bionic on a body part would contribute a small amount of encumbrance, similar to some mutations, except for bionics which are small enough (e.g. Integrated Dosimeter). Each body part would get a small amount of leeway, enough for a few simple bionics, but to become a full cyborg would cause quite a problem and taking each and every bionic would be nearly impossible.
A more complicated but, to me at least, more satisfying and realistic method would be something actually modifying the characters body. As body parts have bionics installed, their health bar would be converted to “integrity” or something along those lines, the idea being that limbs and flesh are being replaced with steel. Integrity doesn’t heal on it’s own or with medical attention, but can be repaired with a welder, or probably far more advanced tools. Balance would consist of requiring a great deal of time or special materials for repairs, or maybe even limiting repairs to special machines found in hospitals and military buildings. Alloy plating would also cease to give an armor rating, and would instead give some free integrity to the part it’s attached to. Steel parts would also be far more durable than flesh, but might be more vulnerable to electricity or acid causing the player to become more of a glass cannon.
The two systems could both be in place, and either one could also include the ability for bionics to prevent mutation. E.G, your superalloy head plating prevents you from growing horns, and they are painfully removed if you install it while having horns. This helps to prevent overpoweredness by limiting the ability to go into both bionics and mutations