UPDATE: I contacted BMWNA about this in the hopes of getting an explanation behind the update tire pressure recommendations. I spoke with Cameron from BMWNA Customer Relations, who informed me that after checking with a tech expert on the M3, the only explanation he could get as to why the pressure might have changed is because around the time the Competition Package was introduced, there was a miscommunication about correct tire pressures and that therefore it's advised to stick to the original recommendations.
Thinking about that after I got off the phone I'm not entirely satisfied with that request. First, recommendations changed for all 3 tire types, not just the 19s, but I thought that might just have been to account for differences in ZCP suspension even if owners fitted 18s or snow tires. But the edition of the Supplement that has the new recommendations appears to have come out in mid-2011, whereas ZCP became available for MY2011.5, production for which started in mid-2010.
In any case, I for one will be sticking to the original pressures.