I think it pretty much would be too powerful. I do have to ask - what sort of fights are you having where you're getting stunned more than two turns in a row? Psi beetle groups or Coil Spiders, perhaps? Seems like it should be quite rare.
The cost of the feat is pretty high, true, but the benefit is huge, too. Getting to take that turn, so you can pop a morphine or a hypo or adrenaline or whatever, is just enormously better than standing around for a turn with no dodge or evasion. Plus, stunning attacks generally aren't as damaging as pure-damage attacks so if enemies are wasting a bunch of stuns on you, they're not doing the harder-hitting attacks which might actually kill you.
I find Thick Skull to be a really good deal for tanky builds; but if it *also* allowed the user to ignore Daze from enemy attacks (while still converting stuns to dazes which did themselves stick) it probably wouldn't be too OP. I certainly find it a less onerous cost than farming Demon Squids for their gel.
