Maybe now is a good time to throw in my own analogy from another sport. In recent years I have begun to suspect that I am going to have to stop hiking in the mountains, which I love, because on the way down my knees just can't seem to take the beating and they are really in pain by the time I get to the bottom. Recently I got some coaching about jogging, where I found that my head position was poor and my weight was not well distributed over my feet because of posture problems. I have been working on this now for a couple of months, and, for the first time since I started thinking about this, I have been hiking again in the mountains. I am amazed that I have been able to come down with absolutely no knee pain, just by paying attention to my posture, my body rotation, etc. The trick seems to be to use my body rotation as a shock absorber, which only works if you have good posture.
So, as Charles notes, just because you get shoulder pain at a given SPL when you pick up the pace, doesn't mean that the pain won't go away if you get some new technique insights. Obviously each persons problems in this regard will be different, but some general discussion about things like body rotation and depth of spearing might help us all to diagnose our problems. The question is, should we be working on these issues at lower speeds or should we also try to figure out what is going bad for us when we start to reach our limits?