It doubles the current boost to the skill mods and reduces only the mdef pierce effect of +11 or higher.
Overall I'd say it's a boost since 10% extra to the skill mod is quite significant, and since there are so many other mdef piercing sources from gear to cards. I imagine average DPS would be more for the skills in question.
It is not a boost actually, lets do an example calculation. Assume it is me or Haseo with Sunflower Boy, Magician's Gloves and +11-14 OBMH, and that our Crimson Rock does 100k damage(which includes all other factors like raw MATK, % MATK, fire magic damage increase, etc). Now keeping all else constant except for Crimson Rock damage and MDEF modifiers, adding 2 x True KK Cards(60% damage increase to Crimson Rock each), and going up against an average monster with 100 MDEF, my result shows this:
Formula: Actual Damage = Base Damage x (1 + 2 x True KK CR bonus + OBMH CR bonus) x (1 - MATK Reduction)
Damage with +14 OBMH(current formula): 100k x (100% + 2 x 60% + 10%) x (100% - 9.64%) = 207.828k
Damage with +13 OBMH(current formula): 100k x (100% + 2 x 60% + 10%) x (100% - 11.05%) = 204.585k
Damage with +12 OBMH(current formula): 100k x (100% + 2 x 60% + 10%) x (100% - 12.41%) = 201.457k
Damage with OBMH(proposed formula): 100k x (100% + 2 x 60% + 20%) x (100 - 15%) = 204k
As you see, my own calculation shows that Crimson Rock gets nerfed so long as OBMH is upgraded to +13 or above, and it gets worse the higher MDEF a monster has. For your reference, when wearing magicians gloves, +14 OBMH gives 88% MDEF bypass(9.64% reduction), +13 OBMH gives 86% MDEF bypass(11.05% reduction), +12 OBMH gives 84% MDEF bypass(12.41% reduction), and the proposed change would give 80% MDEF bypass(15% reduction).
But thats not even the most important point being made here. The key fact is that OBMH has always had bonus per refine level, which has encouraged ppls to upgrade it past +10. Its common to see OBMH at +12 or above. Removing the refine level is a slap in the face of whoever spent billions of zeny or hundreds of dollars to get their OBMH to +13 and beyond. The investment made into over-upgrading OBMH will be completely wasted, I dont even recall Cylinder being treated like this. I know most community headgears' bonus stop at +9 like AHB, but OBMH is a different story and it has been this way since it came into existence. Unless a very good reason is provided, I am afraid this will not make the over-upgraders happy.
Edited by VModCinnamon, 07 December 2017 - 04:08 PM.
Mod edit: Removed response to hidden content.