The heavier buffer has had two effects as far as I can see. First is a slight reduction in the felt recoil. After 50 or 100 rounds, that slight difference does provide additional comfort.

Second, the ejection angle of the brass is a little closer to 4:00 than the 2:00 to 3:00 angle that some of the brass would come out before. S&W carbines tend to be a little over gassed, probably to ensure reliable functioning with lower powered .223 ammo. If you're shooting 5.56, then the heavier buffer makes a difference.

However, having said that, the gun has functioned flawlessly with both buffers, so a heavier buffer is not necessary from a performance standpoint.