I can't be bothered to find my piss-poor old write up but in a little more detail:
the main thing I was testing was whether or not it was worth wearing an extra 2 mail shirts. Thus, I tested mail shirt, breastplate, greaves, gauntlets, helm, shield, and short sword against the same with 3 mail shirts. All combat skills in all fighters were grand master. I cycled through materials, but the whole suit was always of the same material.
the fights were 20v20, with 3 fights per material.
The wiki had informed me that mail shirts covered the most of any proper armour piece and were also the only proper armour piece you could wear multiples of (besides shields), so it seemed like something worth testing in (relative) isolation.
This test was conclusive: the more mail shirts you wore, the more guaranteed your survival. I noticed with searches of the logs that there was practically no difference in amount or severity of wounds received on any part of the body but the upper arms and necks: the only parts covered exclusively by mail shirts. The single shirts would get disabled by wounds to their necks or upper arms, pretty much every time. The single shirts didn't win a single fight.
However, the gap got smaller and smaller the lower-grade the weapons material was. With copper equipment about 4 multishirts would survive each fight, whereas with adamantine the multishirts lost one dwarf once.
And when I redid the tests with spears, it was pretty much even with single shirts slightly in the lead, while with hammers single shirts consistently won 2/3 (Except with addy hammers, where the multishirts dominated again).
When I tried everything again with novice skills, every result was similar but shifted significantly in the single-Shirts' favour. So the single shirts won the copper swordfight, and only lost narrowly in the others (except addy where they still lost significantly) while there was no competition at all with spears or hammers (except addy hammers, where multishirts still won).
Later I did a much less extensive series of tests testing the single shirt setup against the single shirt with max cloth layering. The results were an almost perfect draw. There was no significant trend towards better survivivability among the clothed at all. I only tested iron and steel at max and novice skills, and only leather clothing, but it was still enough to draw a conclusion:
TLDR CONCLUSION:
The best setup is a single mail shirt, breastplate, helm, and greaves, along with a pair of high boots and gauntlets. Clothing does not provide enough protection to bother with. Multiple mail shirts are useless against same-material spears and any hammers, slow down recruits and average soldiers tremendously, and are redundant when a single steel mail shirt is more than enough against any copper or iron (non-impact) weaponry.