I think that there should be an increased chance of a successful hit if the target your are going for has it's back to you, because...it makes sense. I dont know if they already do this or not...
Arrows should hurt...have you ever been injured by a shot arrow? I hope you have not because it would really hurt you. Arrows when shot in the right places can even kill...one well placed arrow can kill in a matter of seconds.
I personally archer in real life so I can confirm that a well placed shot from a longbow can easily kill you, and I'm not sure about 10feet per second? But there is no chance in heaven nor hell that you can move a sheild in the way. But that doesn't mean the shield isnt in the right place anyway.
I learned that long bows can pierce armor in history class a long time a go. So since we want the wellfare of everybody in Illarion we need to make it safer.
At short range an piercing tipped arrow can shoot through a metal breast plate. i wouldn't think it could shoot through a shield but you hardly have enough time to react to a shot arrow unless it's being shot from afar. arrows can quite easily travel 10 fps if used on a 100lbs bow (draw strength) which i believe was the standard strength in those days.
getting back to the proposal i agree that it should do more damage or be easier to hit the person if their back is turned or depending on how close they are (not sure if that already effects it) there should be a much greater chance of hitting someone at close range then from far away. plus ti should do more damage as you can aim at vital organs better.