Alrighty, here's for a test that's probably meaningless! But tests are for testing, and someone's gotta do it!
This test is to see if the hold items, as they require damage calculation, affect the multiplier before they activate (as in, does the Cell Battery/Absorb Bulb/Luminious Moss/etc. reduce the damage taken from attacks of the listed type).
This test is not with the Cell Battery this time, but rather with the Luminous Moss. They should work the same, just with different types. Plus, Water attacks are easier to regulate for the resources I have. Plus the three Pokemon I use are convenient for me.
For this I use a Talonflame, Tyrantrum, and Trevenant (What a coincidence, all T's!), for super-effective, normal, not-very-effective.
Opponent is a Floatzel with Aqua Jet
Anyway basically what happened was; for the no items, I just let him hit me three times in a row, and recorded the values. In the case of Talonflame, I just used Roost each round.
For the Luminous Moss, I tediously forfeited and restarted a battle three times with each Pokemon, because I only had one Moss to use.
THERE WAS AN ISSUE I HAD WITH MY TEST: Luminous Moss raises SpD after getting hit, I used Aqua Jet, it's Physical, obviously, there could be a SpD modifier that isn't there for Def that I did not test, but here are the results for the test on behalf of Defense, and overall weakening of that type (I'm sure you just as much as me know how this played out):
Recordings:
NO ITEMS:
TALONFLAME:
1: 147 -> 81
2: 147 -> 91
3: 147 -> 85
TYRANTRUM:
1: 160 -> 138
2: 138 -> 116
3: 116 -> 92
TREVENANT:
1: 153 -> 137
2: 137 -> 122
3: 122 -> 107
LUMINOUS MOSS:
TALONFLAME:
1: 147 -> 81
2: 147 -> 87
3: 147 -> 87
TYRANTRUM:
1: 160 -> 139
2: 160 -> 139
3: 160 -> 136
TREVENANT:
1: 153 -> 128
2: 153 -> 137
3: 153 -> 136
As you can see, it was fairly consistent. A few outliers here and there but nothing that can't be accounted for over numerous samples, and is acceptable with the RNG.
By now we all know what the conclusion is, but I did calculations anyway.
HP lost:
NO ITEMS:
TALONFLAME:
1: 66
2: 56
3: 62
TYRANTRUM:
1: 22
2: 22
3: 24
TREVENANT:
1: 16
2: 15
3: 15
LUMINOUS MOSS:
TALONFLAME:
1: 66
2: 60
3: 60
TYRANTRUM:
1: 21
2: 21
3: 24
TREVENANT:
1: 25
2: 16
3: 17
Averages:
NO ITEMS:
TALONFLAME:
61.33
TYRANTRUM:
22.66
TREVENANT:
15.33
LUMINOUS MOSS
TALONFLAME:
62
TYRANTRUM:
22
TREVENANT:
19.33
So uh, yeah. Trevenant seemed to have some trouble there at the end for some reason, but the sample is too small to draw large conclusions based off of that. Now if the Moss gave it numbers that were half as high, then, well, there ya go. But at any rate, like I said, before, it could have individual specific stat buffs, but it is not type-damage reduction.
Type-damage-to-stat conversion items do not change the damage multiplier for any--super-effective, normal, not-very-effective--damage types. Through this test, we can only conclude this is so on behalf of reduction of damage from moves of the TYPE listed on the item. This is not to say that these items do not have hidden stat buffs before used (for example, as Luminous Moss increases SpD when consumed, it may slightly raise SpD before consumption), though it is still unlikely.
And that's what I get for 20/20 hindsight in thinking "oh hey, why not use Special moves with it?"
But alas, I did not. And my testing partner did not want to go through another long test again, so that is for another time.
My apologies for the long post, I can't figure if there are spoiler tags on this site! I would love to tag the data so it doesn't take up too much space! Sorry!