This is one of those dumb little pointless questions, but I'll see if I get any answers before it sinks to page 3.
In the Space Marines, we have a nice group of folks called Techmarines. These are one of my favorite character types to consider playing, and I like their options. One little quandary I have, though, is this: Tech Priests seem to resent that they were born of flesh, and as one of the few organizations not beholden to worshiping the God Emperor as a god, but rather as a manifestation of the Omnissiah, they are not required to, in the strictest sense, hold up the Human form as perfection (when compared to numerous Xenos or Mutants with superior abilities), and instead cut out various chunks at the firt opportunity, and replace them with tech, the blessed gifts of their God, in order to be closer to Him (won't the be surprised when they become Necrons?)
Anyway, Space Marines will send their own, with penchants for tech, and a willingness to be alienated by the brothers forever afterward, to Mars, and have them learn the machine mysteries of the Cult Mechanicus, who then return as Techmarines. Now, Techmarines can grab Machine (X), and slowly replace their "inferior" Human flesh with tech, too, but this confuses me. Space Marines are supposed to be like gods, in some ways, and their bodies are often viewed as the closest to perfection a Human can reach. To chop bits off, and replace them with cybernetics, just to follow a religion you kind of don't, seems odd. Add to that, a Space Marine's body is crammed with extra organs. How do you replace most of your body with other stuff, and still have all the extras that make a Space Marine what he is? And if you do, what was the point in laboriously filling him up with extra organs, just to tear them out, and replace them with "better" cybernetics? When I imagine an advanced Tech Priest, I see just enough tissue left to not be an AI robot, feared by all. They have mech limbs, lungs, blood, heart, eyes, armor plating, etc, and might even talk with a vocabulator. I find it hard to believe a Space Marine could do that, and still be a Space Marine. I'm betting they aren't quite as gung-ho about "upgrading" as their Human teachers, but I don't know. I certainly don't remember rules in book saying that they lose to gain, but that could just be to be nice. On a mechanical sense, I don't know how they do it.
If cybernetics can replicate the parts of a Space Marine removed, so that they still have all of their bits and pieces that make them awesome, it's a shame they can't make a tech "Black Carapace", so that other people can benefit from Power Armor, more, but oh well.
As an equally pointless aside, is a Techmarine's servo-arm a part of him, or his armor? The book seems to claim it is a part of the armor, meaning if he removes his armor, say while at the Watch Fortress, or in transit aboard ship, he would not have a SA, and only their own suit of armor would give them access to it. I think that Tech Priests in Ascension (amazing that DH Tech Priests never get one, when they are IG-like, and those guys have one; wonder if OW lets them get one?) have it anchored to their flesh/cyber mantle, as do RT Explorators. Do Techmarines always just have it separate, or might they, eventually, anchor it to themselves? Probably makes more sense attached to the suit, but I is curious.