The Prereqs just seem backwards to me. You need a 15 in either Wis or Cha, so right there, that character already has at least a +2 to will from attribute bonus alone.
The character who needs this feat is the one that doesn't have Wis or Cha as a prime or secondary stat, like the Fighter for example (traditionally low Will saves/defenses).
Seems backwards to me...
And this is exactly why this is not a "feat tax" feat. If there was no prereq, then people would (probably rightly) claim that the feat is so good for the low-Will classes that it's practically required. Thus the "feat tax" nomenclature.
However, with the prereq... it's now a feat for characters who have good Wills to have great Wills. And in that regard, it's no longer a necessity to take, it almost becomes a flavor choice feat. "My PC is so mentally tough that nothing can affect him!". That's why it is good feat design... because it isn't required to take. You can if you want to go from really good to great, but it's by no means a slam dunk choice.
Look at it this way... what if there was no Plate Armor Proficiency prereq that said you had to be proficient with Scale (and Scale had no Chain prereq etc.?) We'd have the same situation. A low Dex/Int character proficient in only light armor would be almost required to take Plate Mail proficiency, because the jump from bad AC to good AC was just so great. (Just like a low Will PC taking this new Will feat). However, a PC with Scale taking the Plate Armor feat is no big deal, because it's a strong AC character getting just a little bit stronger (nowhere near enough for it to be a necessity for the character to take it if he didn't want to.)
So the prereqs for this feat make perfect sense.