The main thing to understand, it that Renfroe is just a much much better player than Katz. There is no doubt that Katz's numbers are very impressive and he should be in the conversation, but for any baseball scout with half a brain, Renfroe is the no doubt better player.
Therefore, we can argue about slugging percentage and RBI's all day, but it ultimately will depend on who votes for these awards. If scouts and real baseball people vote for the award, then Renfroe wins, if the media votes, then we get a Connerly like situation and Katz wins.
Also, do me a favor and stop talking about fielding percentage. Fielding percentage is probably the single worst stat in all of baseball because it doesn't take into account range. i.e. Ozzie Smith gets to a ground ball up the middle that no other SS can get to, and he makes an off balance throwing error. He gets an error for that play when every other SS can't get to the ball and thus its a base hit. Renfroe plays a much more athletic position than Katz and thats not even a question. Also, the expected offensive profile for a right fielder is much lower than that of a 1st baseman. All this = Renfroe much better player than Katz and its really not debatable.