18 HR in 800+ PA doesn't exactly suggest masher to me. 20 HR upside is nice, but hardly "great".
Here's an upside comp I've not seen made that might actually be pretty accurate -- Curtis Granderson, the 2005/6, non-inflated-BABIP one.
I'm really interested to see how his IsoD progresses as he moves up the food chain and is forced to swing more often.