Swap out those AD marks and replace them with Hybrid Pen. Why? Because corki's basic attacks now do 55% AD, 55% AP damage. The logic here is to do more AP damage because the opposing ADC is likely have less magic resist and armor. In addition, corki's Q does magic damage, so it boosts the damage done with Q.

Fairly typical of most ADCs, but I take all points in magic penetration. When majority of your abilities do magic damage and half of your basic attacks to magic, than logically you want more magic penetration.