my understanding is that the difficutly "Expert" maximises spawn rates and enemy aggression, up to the maximum that the re-enforcement rate allows. simply put, they cant respawn troops that they dont have.
both of these settings have variables that can be tweaked and are in the "globals.xnt" file (Program Files\Longbow Digital Arts\Hegemony Gold\Resources\Data). Theyre all in a block entitled <!-- AI constants -->
<threatraiddeterrence>15.0</threatraiddeterrence> <!-- lower values mean fewer units are necessary to deter a raid -->
<managedgoalthresholddropperday>0.2</managedgoalthresholddropperday> <!-- shorter delay between attacks -->
<managedgoalthreatweight>80</managedgoalthreatweight> <!-- level of active threat that deters additional attacks, lower value means fewer simultaneous attacks -->
<!-- recruitment levels -->
"<playerrecruitingbias>0.0</playerrecruitingbias>" looks interesting. on easy its "0.25". it may accept a negative figure for expert if u wanted to give the AI more than yourself. by comparing all the variables in the different difficulty settings you should be able to tweak Expert to your liking, combined with increased recruitment i would imagine you could get back to the feel of PoM.