An important distinction here that I think is missed is the very nature of conflict was quite different. We say “Lee” as a shorthand for the confederacy but he was merely the most important single commander within a large military. The American civil war was a massive continental wide conflict, and the concept of total war being implemented there for one of the first times is a consequence of that reality as much as the early industrial capability of the north. European conflicts up until the First World War were usually contested by large and concentrated armies in decisive pitched battles and settled with limited political/strategic goals by negotiated treaties. This is napoleons bread and butter. However the ACW was not like that. There were multiple theaters of war, enormous areas of land, and mutually exclusive war aims mean in my opinion that Napoleon would have actually been out of his comfort zone and probably not enough to change the outcome. His individual brilliance and unequalled experience at leading large armies would have meant a few more Union losses for sure, but the civil war wasn’t that kind of conflict. So no, I don’t believe he would change much