While steel is suppose to be calibrated for 130 pf often it is not. Unless you hit the steel several times and it doesn't fall your not going to get a reshoot. In a local match they might have you hit it 3 or 4 times call it a hit and tell you to move on. Anyway you look at it your kind of screwed. With a 124 moving at a decent speed you take some of these factors out.
Here is the big thing for me. When I hit something with a 124 it drops with athority. It is also a lot more forgiving with a sloppy hit. A 115 with a sloppy hit may fall but is really slow doing so. If I see this out of the corner of my vision I'm tempted to swing back on it. I don't need that. I want to know that if i hit it, it went down.
At the handgunner the first tombstone on opening doors really needs to be hit several times. One hit might do it but it falls slow even when hit with a 124. Also getting cheated out of a win by sombody powering the stop plate under yours sucks. With a heavier bullet if you do your part this is less of a factor.