Couple things:
First off, Napoli put up 30 HR in 113 games in 2011, and 23 in 108 games in 2012. His 162 game average over the past 2 seasons gives him 40 HR and a .931 OPS.
Playing 1B on a regular basis not only will allow him to play more games and increase counting stats like HR, it will also keep him healthier than having to catch 65-70 games a season.
Second, even in his down year he posted a .345 OBP, and the two year average is a .379 OBP. He grinds out at bats, seeing 4.43 pitches per plate appearance last year, second only to AJ Ellis at 4.44.
I completely agree with UN that he is likely going to give you right smack in the middle of 2011 and 2012. That puts him at .275/.379/.552. Give him 155 games and he will smack you 35-40 HR, especially with his lofted, pull heavy swing.
He's an excellent option to hit in the 4 or 5 slot for the Sox, and having him hitting behind Ortiz will give Ortiz a lot of protection, especially against LHP, who Ortiz hit to a staggering .985 OPS in 2012 and .989 OPS in 2011.
I am 110% all for Napoli. He is, and should be, the Sox top target for 1B and top positional target this offseason.