I don't remember the math, but I had figured it out a few years ago (with help from Aisling). It also kinda delves into a post I am planning on making but haven't yet. From what I remember/ figured the difference between 15% spec bonus and 20% spec bonus is 0.5% to everything spec bonus effects.
In this thread
[HERE] you can see that maxed specializations give you +10% to all your skills (whatever they effect), and maxed diety spec gives you an additional 20% on top of that (roughly, Wiz said it may be higher though) making the total bonus to everything specialization touch 12% (again, roughly)
So you can get an extra 20% on top of the 10% of having a specialization maxed? or is it 20% of the 10% you get for it being maxed (12%)?
Its a relative bonus ie if the max effect is 10% you get 12%, but 10% is not the accurate number for most of the specs (it's higher).
So knowing this Pariah gets 15% forever (Malath too? Or Divine Light) they will only recieve 11.5% bonus to everything specialization touches [This is where that 0.5% difference comes in I mentioned earlier, this post is made in 2 parts so sorry for that]. Because they decided to swear Pariah they will always be 0.5% weaker then someone who is fully aligned with a deity.
This mean:
Blade/Strike Specialization - Improves your skill and damage with 1hs, 2hs, 1hp, 2hp and h2h.
-0.5% less chance of hitting NPCs with these weapon types.
-0.5% less damage from hitting NPCs with these weapons.
Blunt/Agility Specialization - Improves your skill and damage with 1hb and 2hb, as well as your combat avoidance.
-0.5% less chance of hitting NPCs with these weapon types.
-0.5% less damage from hitting NPCs with these weapons.
-0.5% less chance of dodging attacks in combat
Sight/Warding Specialization - Improves your skill and damage with archery and throwing, as well as your ability to resist spells.
-0.5% less chance of hitting NPCs with these weapon types.
-0.5% less damage from hitting NPCs with these weapons.
-0.5% more damage taken from spells (I think with the new resist system? Not sure on this though)
Energy/Defense Specialization - Improves the damage of your damage spells, the healed amount of your healing spells and traps, and your mitigation.
-0.5% less damage from your nukes
-0.5% less health restored from heal spells (maybe lifetap procs as well, or other heal procs)
-0.5% more damage taken from NPCs
Focus/Mind Specialization - Improves your ability to bypass the resists of enemies, as well as your ability to conserve mana on casting.
-0.5% less damage done by nukes (with resist change, still not sure though)
-0.5% more mana used for spells
The difference may sound pretty minimal, but over the duration of a raid encounter or maybe a giant pull in Kaesora, with hundreds or thousands of melee attacks that extra 0.5% mitigation or avoidance or hit rate or whatever really adds up.
This seems like more then enough of a handicap for those who swore Pariah. Lowering the max spec to 10% would mean they have a full 1% less to to everything then anyone else (and the difference for someone like Bango for -20% compared to a +20% spec bonus would be a full 4% difference in everything).
