main issue is that you are letting people ignore 3/5 of an a targets HP.
bad guy has 100 hp? not a problem, you only need 40 damage and he has no arms. unless he is a monk, his feet are only good for running, and then its only another 40 damage until he is just a torso, and a nothing more then a trophy in our "hero's" lair.
also loosing a limb is fairly fatal to anyone without immediate aid.
for an average human i would have the limb damaged in steps, like broken at 1/4 of hp (-2 to limb action, or 1/2 speed), crippled at 1/2 of hp (limb mostly useless, cant attack, or 1/4 speed), with lost limbs being the final step at 3/4 hp.
i would also give the target a bonus to his ac, at least a +2, as i think a limb is at least two sizes smaller then it's owner.