I don't know what to think of this feat. It's just weak.

yeah, it maybe sounds great to wear half plate without stealth penalty, but at what cost.

So we will take a character with 16 dex. Optimal to take the feat.

If you take the feat you have AC of 18. 15 for half plate plus 3 from dex. That is +2 AC upgrade from most probably your previous setup with breastplate and effective +2 from dex.

Now, if you would take +2 dex rather than that feat you would still have 16 AC, but you could have it with mere studded leather.

So, we are comparing;

Feat; +2 AC,
+2 dex; +1 attack, +1 damage, +1 reflex, +stealth(main skill for the feat), +1 initiative, +1 to other skills and thief tools.

and you get to max dex 4 levels earlier so for those 4 levels you have only 1 AC less for all those bonuses.

So,
two solutions:

either raise the max dex for medium armor with this feat to +4 or,

treat it as "half-feat" and add option of +1 to str,dex or con.

Edit;
P.S. studded leather vs. half-plate;
1/17th cost and 1/3rd weight

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