There is no excuse to not try Aphelios AP.

"But he's a marksman!" .. [sg-janna] EVERY ABILITY HAS AN AP SCALING! His Q with Calibrium has a 100% AP ratio. Severum gives 10% bonus movement speed per 100 AP. .. I believe Gravitum and and Infernum have 70% AP ratios, and Crescendum has a 50% AP ratio. And all of his ultimate have 100% AP ratios too. You CAN still place points into AD because he is still a basic attacking champion and all of his abilities still scale with AD as well. On the other hand, with a full AP build though, he benefits from attack speed (again, basic attacking champion), and armor penetration because the only ability that deals magic damage is Gravitum's Q.

Edit: I'm really only saying this because content creators have a bad habit of only trying the most optimum, or at least what seems to be the most optimum, builds on new champions. Very few will release videos of the would-be "off meta" builds before the champion hits live servers.

The biggest downside that I see is that the actual attacks don't apply as much damage so you become much more reliant on his abilities than when building AD.

As for the ability damage, .. Since Severum doesn't have a damage scaling with AP, you don't gain as much health from using it. The movement speed scaling would probably turn this into more of a chasing, kiting, or escape ability.