Does Dex still apply?This gives a maximum AC of 18 for a 17th level fighter in studded leather, which is equivalent to plate. A fighter can still wear plate, of course, which still gives AC when surprised and the benefits of Heavy Armour Mastery if taken.
If it does, this gets you up to AC 21, which is kind of ridiculous.
If Dex doesn't still apply, I think you should make that more clear. Maybe specify that this bonus replaces your Dex modifier for AC calculations.