951,
How much of a loss of extension as opposed to change in overall ROM? The assumption is that pitchers find a way to adjust for the change to some degree but I don't know if there's any way to quantify how much is actually lost. Even if there were some way to simulate the extension loss for a study there's no way to determine if down the road the pitcher would be able to make an adjustment that would get their velocity back.
Eric Cressey considers extension loss something that has to be dealt with, yet I was talking to doctor who works with MLB players who said that many of the MLB pitchers have deficits up to 30 degrees. The studies I've seen show that most MLB pitchers are within the range of 0 to 15 degrees and that there would only be a very few with a 30 degree deficit as that would be 3 SD from the mean. If I remember my stats correctly that's about 1.5 in a thousand so it is actually unlikely that there is more than one MLB pitcher with that much of a deficit, especially when you consider that anything over 30 degrees is considered a problem when it comes to the activities of daily living. Then again stats and reality don't always match.