As far as I can tell, the fans mount to the endtanks which remain the same thickness regardless of radiator. The RS is a 16mm core whereas the WRX radiator is a 27mm core. yamarocket630 swapped his RS endcaps onto a WRX core so he had a WRX core without the mounting tab hassle.
I am not sure what the problem with A/C. I have heard from a couple people that it cannot be retained with a WRX radiator, however I intend to keep mine. I guess it just depends if you want a drop in solution of if you are willing to work for it.
On a little side note, the WRX fans are 120W each whereas the RS fans are only 70W each.