The cost changes barely touched him, tbh.
People just realized that if you don't spam ranged EQ, you don't OOM, and then you don't need Tear, hence why everyone's running him full AD/armor pen now.
Take a look at his mana 'buffs'.
Q hammer change doesn't matter outside of all-ins, and it barely touches his total all-in mana costs. He only ever uses this ability in all-ins, for the most part.
E change literally doesn't matter until the point where he's already fallen off. He maxes E last.
+50 base mana didn't make him any more or less reliant on Tear. It changed pretty much nothing.
Jayce's damage is overpowered because it was balanced around him having a less-than-optimal laning phase due to rushing Tear. Considering people realized he doesn't need it, thus started prioritizing armor pen, his damage is simply over-tuned. He's not broken, he just needs slight nerfs to his early game base numbers. His teamfighting is pretty terrible overall.
Kennen, on the other hand, is actually broken. The change they made to his ultimate over-loaded it in both CC and damage. Go look at the changes. More damage, more CC, less time for it all to actually get laid down. On top of that, he got noticeable laning buffs.