Just did some arena tests. This is in 34.11.
* 5 each of hammer, sword, mace, and axedwarves on each side, making a 20v20 battle. All dwarves were Adept in relevant skills.
* Side #1 had a full set of iron armor, but no cloth/leather gear.
* Side #2 had ONE extra leather layer and ONE mail shirt.
* Ran the test 3 times.
First test, 8 survivors on side #2
Second test and third test, 10 survivors on side #2.
Now, I am laid off so I have nothing better to do than one more test to see if it's worthwhile to go completely overkill on the layers. I did some playing around to put on as much gear as I could without anything coming off. I know this is on the wiki, but here's what I came up with:
* 3 mail shirts can be equipped with a breastplate. I was unable to further add any shirts/tunics with this many mail shirts equipped.
* 8 hoods can be worn with a helmet. Helms/caps/masks cannot be combined in this version.
* 2 gauntlets, plus 2 gloves/mittens seems to be the max.
* 2 boots, plus 2 socks (NOT shoes) seems to be the max.
* 1 pair of greaves will make it impossible to put ANY leggings on. However, 2 pairs of trousers seems to be the max while wearing greaves.
So, for my arena test I made 50 dwarves (10 spear, 10 mace, 10 axe, 10 sword, 10 hammer) with a full iron set, plus 1 extra layer maximum. Then, I made 50 dwarves on the opposing side with as much layering as possible. I know the difference isn't huge: 7 extra hoods, 1 extra trousers, 2 extra mail shirts compared to the initial "one-layer" dwarves.
I ran the test, and group #1 (LESS layering) was victorious with 17/50 survivors. I thought this was weird, so I ran another identical test, this time group #1 had 15 survivors.
Could be a fluke, I dunno.
TLDR: An extra layer gives you noticeable protection. Adding more layers beyond that doesn't seem to help soldiers stay alive though I imagine it's useful for civilian "squads."