JesseH:
It is not necessarily true that 15 inches of penetration will "blow right thru" anybody.
In the real world, your target is not going to be standing still straight in front of you like a target at the range. He will likely be moving, or behind barriers. You may be shooting at an odd angle.
You need all the penetration you can get to make up for this. Not long ago, I saw a man who was shot thru the left arm, left flank, left chest, and the bullet ultimately lodged in the right side of his neck. The only significant injury that he sustained occurred in the neck, after no less than 12 inches of penetration.
Overpenetration does not exist. Other people here will argue this point, and say that overpenetration jeopardized innocent bystanders. Not true, I believe.
Dont forget the rules of gun safety. If it is unsafe to take a shot because of "overpenetration" concerns, then it is still unsafe to take that shot even if your round is an under-penetrator.