My fault, I'd not read all the text on cyber-limbs in some time, and I remembered that just having a limb made you harder to kill and assumed that was armor. But on actually going and reading the book, it is 1 extra box on your damage track, not 1 extra armor. Sorry for any confusion. (I hadn't really been looking at a tank, more of 'what sort of swiss-army-knife of toys could I pack into a leg?' so I'd not yet gone into serious review of all the rules on that)
ETA: while I have cyber-limb experts reading -- I don't think there is anything in the rules that says too much chrome makes you sink like a stone? (in fact, if the limb(s) average up your strength, you'd actually swim better !?!)