The WRX came with a 10mm pump, the STi 11mm and now the latest ones have a 12mm pump.

The 11mm has the best oil flow (about 20% more) and a slight pressure improvement over the 10. IIRC The 12mm is a higher pressure pump, but doesn't flow as well as the 11mm.

Theres discussion about the benefits, but concensus is that the 11mm is a smart upgrade.