I asked the one of their Wi-Fi nerds about this and I was answered with a big grin. Apple does not publish this. You can get an ideas of the power output from the FCC. But you will not be able to get official RSSI to data rate mapping from Apple.
Seeking for FCC testing reports is a good idea!
In case you do not know where to start, look for FCC ID at device and then search FCC database at: http://transition.fcc.gov/oet/ea/fccid/