though they're also using Rails 3.2.11, which not only isn't the latest 3.2.* release but doesn't take advantage of Ruby 2.0... it would be more interesting to see the latest Rails 3.2.* on Ruby 1.9.* along with the latest Rails 4 release candidate on Ruby 2.0.0-p0
I encourage you to benchmark Rails 4 yourself and see if there is a measurable difference from the latest 3.2.x, to get a preview of the impact it will have. When Rails 4 is released we'll definitely want to upgrade to that.
Edit: pfalls is too fast for me, and just completed the upgrade to 3.2.13 and closed the issue. The next round will use Rails 3.2.13.