I'm gonna go out on a limb and disagree. Clearly your team is pretty good, but I think you'd be better overall with a little less speed and a little more power. You have over 200 SBs (using last year's stats) and I'd guess that 150 is plenty to take 1st or 2nd in a 12 man roto league. Therefore, I'd try and trade one of your big SB guys, Figgins, Rollins, or Podsednik for a more power oriented guy. I especially like Figgins for Utley, or Rollins for Jeter if you can get it. I'd probably even take Rollins for Peralta. Anything of that sort. Try to give the SBs to a guy who doesn't pose any kind of threat to you (probably most of the league falls under this category, but surely someone has a chance).
As for upgrading a closer...screw it. They'll want offense from you to do it, and it's not worth it. You dominate offense all around right now, and giving up some of that puts you in more jeopardy than a couple saves is worth. Take your low saves score...you'll win anyway.