Sacks aren't a real gauge of how well a player is doing even for pass rushing specialists. To get a real picture of a players dominance (without breaking down film) you need to look at defensive pressures, total hits on qb, deflections. For instance a player that records 20 sacks, 30 total hits on qb & 2 deflections will get a lot of press, but the player that only records 11 sacks, 60 total hits on qb & 6 deflections will be the guy offenses fear more.
Now in terms of impact, Aldon Smith is the reason why Jim Harbaugh is a great coach and Mike Singletary was labeled a bad one and fired. The 49's could stop the run with the best in the league prior to his arrival, but they could never get off the field on 3rd down against quality offenses because they had 0 pass rush. Smith came in last year & changed that immediatley, which placed Justin Smith and other DL's in their proper secondary role. Because of that they were a few bad plays away from making the superbowl despite Alex Smith's horrible play.