Instead of all of these crazy Soraka changes...

Why not make her Astral Infusion (W) cost 45 + 4% missing mana, with her health cost untouched? This will cause her heal to not be spammed at every moment it is up, as her mana costs will drastically increase - and this also is a nerf to long-fought mid-game teamfights for Soraka, as she won't be able to keep up with her heals and silences due to her mana costs.

TL;DR: Make Soraka's Astral Infusion cost scale with missing mana; this will result in less spammable heals.