The new 802.11ac AirPort board while it fits and in fact works there is a bit of a problem: The antenna's.
The AC spec requires two identical antenna's, your system does not meet the AC requirements. The bottom line here is you can't get the data rate you are hopping for with the exchange out. Why is 802.11ac not in the MacBook Pro Retina. While this is on a MBPr the Air is really no different.
The other issue here is the availability of the board. Remember this is a Apple custom part! Apple's own need for building new systems will make getting this part almost impossible. Apple requires part exchange outs for this part (they want the bad 802.11ac part part back) Basically, Apple is controlling the availability of the part even as a spare.
With the upcoming models likely to have 802.11ac support we're looking at mid year next year when the pressure will subside.