What does it help in game terms to impale him harder? Do you allow a bonus on the potency roll for that?
Also, I don't know whether that makes sense or not. If you are hit by a needle covered in something nasty, if it gets into your blood, it gets into your blood. It doesn't matter how deep the wound is. And the limiting factor is probably more the amount of poison on the needle than the amount of needle that's stuck in the body.