According to this source who saw live A5 MacBook Air actually, this test machine performed better than expected.
Though it's not clear which Mac OS X or iOS is pre-installed on this A5 MacBook Air, iOS seems to have difficulty to use features of Thunderbolt without Finder. And even if Mac OS X is installed, developer should spend time to support A5 on Universal Binary Applications. As considering these situation, this A5 MacBook Air seems to be made just for experiment.
A recent report from SemiAccurate said Apple plans to switch its notebook line to ARM based processors in the future and although it won't happen soon the site claims "It is a done deal". The short story is that Apple is moving the laptop line, and presumably desktops too, to ARM based chips as soon as possible.