I'm going to go along with Secret Avatar here. "Can" MCab hit 40? Yes. However, to do so, I think he's going to need a slight change in his approach at the plate and, until he does so, I think hitting 40 will be tough.
"Most" 40+ HR hitters are FB hitters with GB/FB ratios less than 1.00. MCab has consistently been around 1.2. There are exceptions, of course (e.g. Ryan Howard), but MCab does not strike me personally to have the same type of skills (i.e. pure power) such that he'll hit a much higher # of HR/FB as compared to a player like Howard.
Throw in that park where line drives that are hit 350 feet hit a wall instead of clearing a fence and barring a little more loft on his swing and/or a new park and I think 40 HR will be tough.
That said, a great player and I would fully expect a .330/30-35 HR player. And, with a little more experience, I won't be shocked to see double digit steals (obviously he's close with 9 last year) - a nice little bonus.