Overwinding depends on the way the mainspring is wound against the barrel. In automatics that I have personally looked at the mainspring has been like this one picture (Picture orginally posted by RBJ).
You can see at the end the spring forms a Y. This Y provides resistance against the side of the mainspring barrel so that the spring can be wound. At some point however, it can slip. So you would not be able to overwind this mainspring as it would just slip at some point. On the other hand some mainsprings have a small indent or puncture that attaches to a piece of metal on the barrel. On this type you will feel the winding stop when the spring is fully wound. If you continue to wind the movement by forcing it beyond this point, something has got to give. Most likely you will pull the mainspring off its attachment point, but something else in the winding mechanism could be damaged also.
As far as power goes. The mainspring has more energy when it is fully wound versus when it is close to it's unwound state. It is the job of the escapement to regulate this energy regardless of the diminshing power of the mainspring as it unwinds.
