power compression is only one part of the story. you need to look at RMS MMS and QES and also the alignment. think about this. acoustic dispersion and acoustical loading can cause give you phase issues and decrease power handling.
basically thats like saying them 1 12 can handle more power than the 18 but without knowing at what acoustical limits and varibles you may have.
lets put it this way most of the loudest systems use fewer larger drivers. phase plays a big role and well. for example lets say i can do 2 15s passenger side over and under and place my power the appropriate distance to avoid loading hte drivers in a non linear fashion while providing the rear wave the needed distance to have perfect phase at the dash. this setup will be more ideal than 4 12s in hte same configuration simply because of phase and loading issues.