Synner on Cyberlimbs (Source)
Stacking Armor - We could have been clearer but it was assumed by the designers that all types of cyber and bio armor enhancement were understood to stack with worn armor. Not making sure this was clearly conveyed may have been a mistake but the rule itself is not unbalanced given the limitations of Capacity and overall costs of both enhancements and the cyberlimbs themselves (in Essence and nuyen). Yes, you can build a (relatively) bulletproof character, unfortunately there will be little room for any other enhancement in his cyberlimbs and it will show in his Atttributes. As has been stated in this thread, being (relatively) bulletproof isn't the gamebreaker people assume at face value (particularly not compared to some of the feats magicians are capable of and not at the cost it comes at).