Its a GM call at the end of the day. If I were running the game I would require the full limb. Aiming anything SMG sized or larger is going to involve elbow and shoulder articulation in both arms.
Honestly, Pistols really should require both limbs as well. Its very cinematic to allow someone to fire pistols one handed, but two handed stances like Weaver or Isosceles are vastly more accurate. In the name of good drama I would probably let that slide, however. In general though, I find the cyberhand approach to be tending toward the cheese, and would allow the cyberlimb attributes only on checks that used only the hand, such as a grip strength test, or a agility test that only involved the hand and fingers. Lockpicking comes to mind, but if video game play or typing skills ever come up in a run your 9 agility hand only guy would be completely set.
