That is exactly what I was wondering. Barrs stock supposedly took a hit after the combine and Mack's improved/stayed the same. I thought Mack looked good and in games he did everything successfully. He would be an incredible weapon for Williams. He has been compared to Ricky Jackson. He is a very strong pass rusher and plays the run well. He may not be good at dropping deep, but he covered well near the line.
That being said, he is a luxury for the Rams at this point IMO. They have more important needs.