Joe, I have close to 2,750 rounds through my M&P in just over a year. The first time that the ejection has been anything but consistent was when I shot a steel challenge match this weekend. I had one piece of brass ejected back into my face. All other 2,749 rounds have ejected to 5 o'clock and clustered together very well.
In short, don't buy a new extractor yet. If it ain't broke, don't "fix" it.