Doubtless the MBP13's performance will be much better, thanks to the fan. Basically it will have all cores (CPU and GPU) clocked higher.
Granted, they could have just left the fan in the Air and accomplished similar results. Seeing as the Air seems to sport the same chassis as before, it does look like they just pulled the fan out to save money and force it to throttle more so the MBP13 could be distinguished in some way.