anybody can tell me the perfect PL that can match if it running about 620 640 cl4 ?

soon there will be a reguler OC competition in my town, and i'm thingking to bring my sticks into setup

what is the best vdimm if you bench it around 5-6 hours with bandwith 1150-1280 Cl4, one of my friend ussualy gave 2.8V to his team extreem, to give an ultimate stabilize , but with my sticks can it go lower ? thx..