I'm not sure what to think about the impact of Clemons (and by extension, Irvin). Yes, Clemons has a ridiculous number of sacks over three years, but let's be honest...this team has almost zero pass rush. It's barely there. Despite Clemons. Consistent, every-drive, truly QB-disconcerting Giants-style pressure is not present on this team and never has been under Pete Carroll.
I can't help but feel, then, that Clemons' impact is more coincidental than absolute. In 2010, Clemons was still getting sacks, but the team was getting steamrolled. Clemons' improvement certainly doesn't SEEM to be the piece that has transformed this defense.
I might guess that other players are getting Clemons opportunities. I couldn't say whom. Now obviously, someone still needs to get those sacks, so Clemons is nowhere near worthless. But the fact that we have the #1 scoring defense in the league despite the absence of pass rush (yes, you read that right) does seem to make the DE position a little easier to reload.