Around here at Axure HQ, we use MacBook Pros for our work. We treat them like development machines and use them for every aspect of our workflow, from development through support. One of the major things we demand from our computers is the ability to run a Windows VM, which is especially crucial for the support team so that we can duplicate all sorts of user environments. We’ve found that the MacBook Air can’t quite keep up with that side of our work.
That being said, none of us has had an unsatisfying experience using Axure RP on a MacBook Air and moving from a 2011 model to a 2015 model will bring a significant performance boost to your workflow. We’d expect to see totally solid performance on the newer machines.