Zac, contrary to popular belief, has a ton of burst damage. His Q and R have 50% scaling and his E has 70%, his W also scales surprisingly well. Roaming to other lanes will almost always result in a kill.
As for me, I have level 5 Mastery with Zac, and it's mostly because of playing AP Mid Zac all the time. It's insanely fun and he's honestly the most fun mid laner I've ever played.

Again, Zac is a very strong solo laner, and giving him ap only makes him better. Since you are going top however, you will be going less damage and more tanky. But don't worry, you still do a butt-ton of damage.

There honestly aren't very many problems for Zac, champs with hard cc lockdown are obviously a problem, but as long as you dodge their initial abilities, you can all in them or E away.

The only reason you might have tried it is because recently /all chat featured a video of an AP Zac dominateing a game. This is AP Zac every game. Make it happen