So it's a -2 to hit if I go for slicing off an arm?
No if you wanted to attack their weapons and destroy them etc. you can take a -2 for a large weapon (like a rifle or big wrench) or -4 for a small one (a pistol, etc.)
There is a talent that allows you to cut off a limb instead of killing someone (if you reduce the targets hit points to negative). I am considering allowing anyone proficient with lightsabers to do it...