His goal-scoring numbers were actually better when he was playing with Florida for a perennial cellar-dweller, so your argument doesn't really hold much water.
(Also, I can't quite tell if you're trolling the board, jury's still out.)
His goal-scoring was higher because he played almost 3:00/game more with Florida than with Boston. His actual goal/assist/point rates per 60 minutes of TOI were actually all better with Boston. 1.11 G/60 vs 0.757, 2.22 P/60 vs 2.03, and 14.07 Shot Attempts/60 vs 12.98.
His PP numbers are better with FLA, but still middling when factored against the rest of the league. Horton had 3.40 Powerplay Points/60 in FLA vs 2.48 w/ Boston - but his FLA PPP/60 was just 181st in the league over that time.
So with Horton what you're getting is a secondary scorer who was put into a primary scoring position with Florida. Yes, those teams were bad, but using Nathan Horton as their top scorer didn't help. If he can be counted on to be a 25 goal/50 point guy in Columbus, but not "THE GUY", then I think the contract should be okay, provided his wonky shoulder stays where it's supposed to.
Is he more than his numbers indicate? Sure, numbers paint a nice picture, but they don't tell you everything. What others have said about him in this thread hold true - he's a big body, works hard, positive influence on his teammates, and if used properly will be a great asset to the team.
But if the team is putting their eggs into the "Horton and Gaborik have to be the best players every night" basket, it's asking for trouble.
Edit: I'm hoping that with all of the other talented players on the team, this isn't going to be the case - many are already slotting him in as a second-line option, and the team has a third line that can also put up some points and play well. If that's how he is going to be used, then this deal may well work out.