top mage supports?

If you want to do a ton of damage, play a role that's supposed to, because clearly support isn't the one for you.

Seems you want a mage support with mobility. Your best bet would be Lissandra, but LeBlanc would also work. I’ve seen both perform well in the support role on multiple occasions. In fact, support LB was somewhat common at one point.

Leona and Pyke work well as high-damage supports, and Morgana has a surprising amount of damage on her Q. You also have Sona and Nami for more poke-heavy supports.

I generally suggest against true mage supports though - they tend to offer little to their team besides their damage, which means if you don't win lane with it you're going to remain useless for the rest of the game, whereas most other supports (even the highly aggressive ones) bring some form of utility, reliable CC, or peel, whereas a mage support is just going to be a shittier version of their already existing mage, and will hurt their existing mage's effectiveness by encouraging the enemy team to buy more MR.

Now, if you get ahead on one, you're likely to just snowball the game out of control as you effectively remove two of the enemy team from the game while likely being ungankable due to your high burst. But the odds of that happening are significantly lower than the odds of you not doing that.