Yeah, by hiding this under the BB AP vs DD change, it's easy to avoid discussion about it altogether! Clever!
I oppose both changes. The BB AP vs DD issue is the more minor one, and since the current DDs are far from unplayable after certain radar nerfs, I simply don't see the problem in the current damage mechanics. However, I especially hate the exclusion of two ships from this mechanic. This is plain stupid, and would be the first time a specific ship is excluded from a universal damage mechanic. This only confuses people, and is in no way needed to nerf these two ships. It also creates a predecent to single out ships and make them fundamentally work differently. The current varying HE penetration formulas, spotting & radar ranges, and hidden (still) sigma & krupp values are an issue themselves. There is already a lot for the average player to memorize and learn, introducing separate damage formulas for different ships is not the way to go.
If you consider Harugumo and Khabarovsk overpowered, why did you introduce 1/4 HE pen for the whole Akizuki line? Why did Khabarovsk already get nerfed in range and rudder shift, while the easiest solution would have simple been to nerf its ludicrous speed. The ship, with the belt armor and 4 turrets was never intended even in "sekrit dokuments" to go that fast.
It sometimes seems to me that when nerfing / buffing ships, the developers rarely go for the obvious choice. Hood suffers from slow turrets and inaccurate guns? Lets make the turrets heckin' (unrealistaclly) fast, and leave the guns as is. USS West Virginia is a famously rebuilt thicc American BB? Lets make it a pre-Pearl nerfed tier 6 version instead! etc. etc.
The 10% damage vs bulges issue, however... I don't even know where to start.
If there was an issue with the no-damage penetrations, why not simply call them such? Make a new ribbon, called "module hit" or "bulge hit". The fact that you simply change the damage mechanics instead of fixing the UI, is mindboggling. Now every signle small pew pew gun could damage a fully armored battleship, shooting its thickest armor, is laughable. 200 damage per AP hit, that x 5 or 6 on high tiers, that is 1500 damage every 4-5 seconds. With literally no skill involved by aiming at the parts of the ship that you actually should be able to damage. Just aim at center of mass.
Another can of worms you are opening are the numerous premium ships suffering from this. While you have excluded ships from purely blanket nerfs before (Gremy & Kamikaze gun damage, Kutuzov range with AFT nerf etc.), this change is NOT even a blanket nerf. The stealth firing nerf was a blanket nerf, it simply meant that no matter the class, or range, or gun caliber, stealth firing simply was not a thing anymore. It applied to premium ships too, ofc. But now you are introducing a mechanic that BUFFS ships with no bulges (looking at you Mutsu, Ashitaka, Kii) and makes their armor BETTER than the ships... bear with me... that had *gasp* an EXTRA layer of armor, in front of their main belt. In many cases (tier 8+) the bulges in the middle are thinner than the extremities of the ship, and sometimes cover the whole side of the ship (RIP Amagi). Now it suddenly becomes easier to deal damage against the previously thickes part of the ship, because now it is suddenly the weakest part of the ship.
Lets take an example, a Panzer IV tank. Later models had these large sideskits covering the tracks and also extra plates surrounding the turret. Meant to deflect AT rifle rounds and to detonate HEAT rounds before they reach the main armor plate. Now, with your current mechanic, the tank would take damage from everything that goes through the plates, even if literally nothing of importance (in this case, just air) is between them and the main armor.
In warships, the torpedo bulge acts the same way. It is meant to detonate the torpedo warhead, and while filled with both air and water, protect the main belt from damage by providing a buffer zone. Now you introduce a change where simply by shooting a part that has nothing in it, causes damage.
I really hope that you think this change through, and at the very least separate the external bulges from it.