a) you can't get them anymore, as the currently sold h/w revisions are Broadcom based and unusable.
b) This is incorrect, yes - you can flash a wrong firmware image and make it show the c7 markings/ only accept c7 firmwares, but that doesn't magically make the 5 GHz hardware use three streams instead of two, nor does it touch the calibration data. --> pure placebo effect, without functional changes or improvements.