I watched the highlights from most of Tulane's games this year. Obvious caveat is that these are the highlights.
I like Mensah a lot. He clearly doesn't have the arm strength of Murphy. Still, he is capable of hitting deep balls on occasion. More importantly, he's capable of making a lot of different types of throws. He spreads the ball out to a number of receivers and throws a soft, catchable ball. What he lacks in arm strength he makes up for significantly in touch.
Now, his legs. He may not be Riley/DJ/Sirk in terms of running, but he may seem that way in comparison to the immobile Murphy. Tulane rarely called designed runs for him, which was curious to me. He can clearly run. Almost all of the yards I saw came in scrambling situations on called pass plays. I think if Duke wants to sprinkle in some read option, he can clearly handle it. Also, he is good at moving his feet in the pocket. He can avoid the rush to make plays with his legs or arm. I like his ability to scramble in the pocket, but still get his feet set and shoulders squared to make throws downfield. Also, Tulane did call a lot of designed rollouts for him.
I'd say if we ran an offense similar to what we ran under Cut and Elko, Mensah could easily double his 132 rushing yards, if not more. It remains to be seen whether the current staff wants to use the QB in the running game that much. At minimum, his mobility should help extend plays.