I think Kerley will be an upgrade over Mason in the slot. Just like playing corner on the outside is a different animal than on the inside, playing receiver on the inside is a different animal than on the outside. Mason has been on the outside for most of his career. The route technique is a little different out there.
On the outside, in almost all of your routes you want to press vertical in order to try to make the corner turn his hips so that its easier to create space out of your breaks. The technique is to drive hard off the snap, drops your hips at your spot, then explode out of your break. Its more vertically oriented. I think its different in the slot.
In the slot, the DB has safety help directly over the top. So the DB is less likely to be forced out of his back-pedal and he can sit more on the shorter stuff. Therefore, the slot receiver has to be more horizontally oriented than vertically oriented. He has to be excellent when making his cut. So good that the DB has to try to guess which way he's going to break. Kerley can do this. And he can do this well. Mason may be a dinosaur with all the experience you can asked for. But he made his bones on the outside. Kerley made his bones in college in the slot.
Kerley also gives more flexibility to the offense. In college, in addition to being a very good receiver, he was excellent at WR-screens, running reverses, and running some wildcat. By having him on the field while running more traditional plays, it makes it easier to go to the gadget stuff without signaling it the way we did with Brad. We can go from a 3-wide look and then no-huddle to the wildcat right after a completion without changing personnel.
I admit that the depth at receiver is a little scary. But I think that having Mason and trying to get him incorporated was hurting the development of not only Kerley, but also Joe McKnight. And he may have even prevented some of the creativity involved in scheming for Keller. Mason may be the better receiver, but Kerley might be the better fit. Not just in the slot, but for the team overall.