I don't really see Mercilus as a viable option as a stand-up player. He's more of a true LDE in my opinion.
As for his ability as a pass rusher, most of his sacks are due to effort and being relentless. They come when he's already passed the quarterback, and has to work back to the QB. His sacks come on plays that are long developing plays where the quarterback is still holding the ball. That's why he has so many forced fumbles, and a lot of those came when he had a back trying to block him (see Arizona St., etc.).
Nothing about him really stands out in terms of rushing the passer. Average 1st step and initial quickness off the snap. Doesn't show any pass rush moves (spin, swim, arm over, dip-and-rip) He's not even getting them on pure athletic ability or length.
What he has is an outstanding motor and plays with pretty consistent leverage. I like Andre Branch better, and I'm not much of an Andre Branch guy.
Mercilus doesn't compare to Nick Perry or Courtney Upshaw in my opinion.