There's no reason you should ever play Jayce
This champion scales horribly, making him decent until you hit mid to late game, has a high skill cap, huge mana costs, doesn't even have a real ultimate for pete's sake, can't win most 1 to 1's without jungle help, and is basically a poor
man(or woman's) Elise. You can play him mid, top, or even bot and you will find him outclassed.
Did I mention he's completely useless if he falls behind.