I stopped using phasewalker on all of my masteries except Ryze. Never really noticed or cared for it. Might as well try something new. I also decided to copy your idea and use expose weakness on one of my mage masteries. :)
I am actually not using Perserverence on AP Tank Masteries anymore. Since I look at Malphite, or Singed, or Cho Gath and say to myself. "These guys are going to run out of mana and make the Perserverence useless."
As for Singed, his base HP is just so high so he has natural health regeneration. I like to use extra magic resist because AP tanks are generally weak to special attackers.