It wouldn't necessarily improve everybody. Typically, the greatest iron players I ever saw were very straight with the irons. However, somebody like my friend Tony from back home in NY hit push-draw after push-draw and was constantly flagging shots and is easily in the top 20 iron players I've ever seen. I wouldn't change his draw shot for anything. But you have to figure that his face is probably around 1-2* open with a path about 3-4* inside-to-out.
The problem is that for most golfers they don't have a face and a path like that. If they typically hit a hook, their face may be 5* open with a path 10* inside-to-out. Tough to consistently do that. So if they try to work on 'zeroing out' their numbers, they may not wind up actually zeroing them out, but get into a more manageable range like my friend has them at.
3JACK