I participated in several SAD procedures as a med student. Did the doc say you "might" have a bone spur which is causing the impingement or did he say you "do" have a bone spur visable on X-ray?
Basically, aside from anatomical abnormalities, the most common cause of impingement syndrome is that repetitive activity causes breakdown in the rotator cuff muscles and results in tendinitis. The resulting weakness in the rotator cuff muscles causes loss of effective movement of your shoulder joint. This causes impingment of the cuff muscles under the acromion (pinching muscle against bone), enhancing the pain and inflammation.
Here is some info from my PDA for you:
In those <25 years old, impingement is usually related to laxity caused by instability.
In those 25 to 40 years of age, impingement is usually due to overuse of the rotator cuff.
In those over 40 years of age, it is caused by use of the cuff muscles over threshold. This may result in partial or full-thickness tears, in additioin to impingement.
Anterior acromioplasty (also called decompression): the acromium is "shaved" to allow more space for the rotator cuff. It is used only if conservative measures fail. There is a less favorable outcome in younger (50% success rate) than older athletes.
I think that if you have an identifiable anatomic abnormality (such as a bone spur) which is creating the impingement, the probability of success would be greater than the normal rate. This is because the normal success rate includes all SAD procedures, many of which have no known culprit for the impingement prior to surgery. If you have a known bone spur, chances are it is the problem and decompression would fix it.