Ok. Will be posting test results here - will edit my post. I'm testing against fully bronze-clad goblins, which is not a realistic scenario, but is a middle ground between copper and iron, it's sort of an average.
10 maple-clad dwarves with metal_obby short swords + glumprong shields
vs.
10 bronze-clad dwarves with bronze short swords + bronze shields
Dwarves got slaughtered. One goblin dead, the rest have no large injuries. All 10 dwarves died quickly.
20 maple-clad dwarves with metal_obby short swords + glumprong shields
vs.
10 bronze-clad dwarves with bronze short swords + bronze shields
Dwarves still got slaughtered. Two goblins dead, the rest have no large injuries. All 20 dwarves died quickly.
From those tests it seemed that the obsidian swords were doing at most some bruises, occasionally clipping teeth and fingers, while the goblins were happily lopping heads and limbs off with ease.
10 Dwarves with all adequate combat skills, maple armor and obsidian swords, glumprong shields
vs
10 Bronze-clad goblins with all novice combat skills
Dwarves still lost, but put up a lot better fight. 3 goblins died, the rest with heavier injuries. Incoming damage seemed the same, but the more skilled dwarves were able to score a few more headstabs and limb-cuts before getting decapitated themselves.
20 Dwarves with all adequate combat skills, maple armor and obsidian swords, glumprong shields
vs
10 Bronze-clad goblins with all novice combat skills
Dwarves won with no casualties. They did have some major injuries but it looks mostly recoverable. It was a rather long fight since once some goblins got knocked unconscious and then for a few days the 10 dwarves repeatedly kept slashing at the head and getting deflected, until one lucky hit severed the head.
10 Dwarves with all adequate combat skills, bone armor/leather supplementation w. bone crossbow/bone bolts
vs
10 Bronze-clad goblins with all novice combat skills
Dwarves got slaughtered. again. With only bone bolts they bounced right off the bronze armor, and once got up close they had no chance. Behind fortifications, they probably wouldn't do much since the bone bolts simply can't penetrate metal armor well.
Ok... this testing was fun, even if it was of little use it was still pretty cool to watch, and I've come to the following conclusion:
If you're gonna take on a metal-equipped onslaught with only bone, leather, and wood, your best shot is to outskill and outnumber them with at least 2:1 and the strongest armor you can. Ranged weapons hardly work due to the lack of penetration with bone bolts.