Napoli's definitely a sleeper, but he has been for the last couple of seasons even though he'll project to be a late-round flier. He has a good power stroke to suggest he can be a 20, even 25 HR catcher in his prime. His plate discipline has been hit-and-miss. Napoli tends to strike out a good bit and in other PA's he can work counts patiently enough to draw a walk. Angels management have been hopeful he can take strides to improve his plate patience.
As far as playing time goes, I think Scioscia will opt to split time between Napoli and Mathis, with the bulk of the starts going to Napoli, but is the better of the two to nail down a 5-days-a-week job.