The MacBook Air from 2012 or later can drive two 2560x1600 displays at 60fps: https://twitter.com/Antagonist/status/252936243606859777/pho... (It only has one Thunderbolt port, but Thunderbolt displays can be chained.)

While the integrated graphics on the 2013 models can drive 4k displays at 60fps, only Thunderbolt 2 has the necessary bandwidth. The latest rMBPs have Thunderbolt 2, so they should be fine once OS X's drivers are updated. Everything else is stuck at 30fps.