TTLG Forums visitor Walton Simons (how appropriate) has written up a nice couple-pages report derived from all of the available information present in Deus Ex regarding the Series P agents. Most interesting, probably, is conflicting reports about their internals, and mention that the P agents, according to in-game texts, are allegedly not modified. Interesting.

You can read that here. And by the way, if you saw anything in the game about MIB agents having an air filtration system in their augmented bodies, stop by this TTLG forums thread and help Mr. Simons out with his report. He is looking for this rumored information in the game.