I was a bit shocked when the pads arrived as they are a bit bigger than your average marine pad. I was going for lopsided from the start though as I like the old fluff snippet somewhere that says marines left shoulder pads on a specific mark armour (I forget which) are reinforced as that is the side they are trained to present to the enemy whilst firing.
The fact that the sternguard shoulder pad is even bigger than a regular marines expands on this idea and as mentioned above gives my arbites a quasi shield that protects a lot of his torso.
there have been a few mixed responses to the shoulder pad across the forums, some love it, some not so much
having paid for 12 of the pads in one go i will be using them on all my arbites, waste not want not