We've been through this before over several seasons. Base AD nerfs are big for Corki and his last base AD + AD growth nerf nearly killed him. He offers no utility so his performance is very sensitive to these changes. Only a succession of ability buffs led to him being meta (at least in pro play).
The PBE changes are a massive mistake and it's not even close to leading to a balanced state. It will gut him. He may at first get some more picks in competitive due to inertia but he'll be terrible (especially so in SQ). He's already a non-factor until he gets TF-Sorcs and these nerfs hurt his late game scaling identity too. In fact, he suffers more than most ADCs because he also builds Sheen and has decent ratios on abilities.
He either needs compensation elsewhere or no nerf to his AD growth stats. In fact, nerfing his base AD by 5 and adding to his AD growth stat to compensate for it late game is probably sufficient. Buffing counter champions such as Kassadin will already hurt him too. Start from there and see if more is needed. Why ignore Corki for weeks and then suddenly hammer him into the ground till his face is disfigured? It's even more ridiculous considering he got several buffs in a row which could just as well be reverted instead of this. The proposed nerfs are probably bigger than all the recent buffs combined since they attack the heart of what he's supposed to be in a team comp.: a scaling DPS carry.