It's a fairly heavy tire, according to TireRack's specs. 27 lbs each in my size, as compared to 21 for the Kumho's I was considering.
According to Wiki, silica is used for heat resistance in tires. At Pirelli's US site, the P4 is listed right underneath the new PZero under the Innovation tab with a date of last revision as 04-09-07.
I shot them a message via their site asking if it is indeed a low rolling resistance tire. I'll see if they respond.