I know this sounds like a slightly odd question, but given the vast array of armour over the generations of metal armour from the 3rd century BC until the common use of gunpowder weapon beginning in the 15th century.
(I understand the scientific method, as we understand it, didn't really appear until really the end of the medieval period.)
Was there any attempt to performance test armour? Or was it a style thing? Was there comparisons like during WWII, like how armies tested or took apart enemies and allies tanks and planes?
Did historians comment on the benefits or disadvantages of their own 'nations' or enemies armour?
p.s. I am aware that metal armour was used until really WWI
There was performance testing. A common "proof" (i.e., test) of armour was to shoot it with a crossbow, until firearms became common, after which shooting it with a pistol was common.
Ffoulkes, Charles (1912), The Armourer and his Craft (Dover reprint 1988) has a chapter on the proof of armour, with quotations from primary sources. There's a longer discussion (but in French) in Buttin, Charles, "Notes sur les armures a l'epreuve", Revue savoisienne (1901), 61-92.
Most of the written evidence we have for proofing armour is 14th century and later. The only evidence I'm aware of for ancient armour is from Plutarch's Life of Demetrius (section 21):
For his use in this war there were brought to Demetrius from Cyprus two iron coats of mail, each of which weighed only forty pounds. Wishing to show their strength and power of resistance, Zoilus their maker gave orders that a catapult's missile should be shot at one of them from a distance of twenty paces, and in the place where it struck the iron remained intact, although it did get a faint scratch, such as might be made by a graver.
While this was certainly a test, it might not reflect common practice.
Bashford Dean, Helmets and Body Armor in Modern Warfare, suggests (pg 39, paraphrased here):
And Saul armed David with his armour, and he put an helmet of brass upon his head; also he armed him with a coat of mail.
And David drew his sword upon his armour, and he assayed to strike at it: for he had not proved it. And David said unto Saul, I cannot go with these; for I have shown that they were not proof. And David put them off him.
as a better translation of 1 Samuel 17:38-39, usually given as:
And Saul armed David with his armour, and he put an helmet of brass upon his head; also he armed him with a coat of mail.
And David girded his sword upon his armour, and he assayed to go; for he had not proved it. And David said unto Saul, I cannot go with these; for I have not proved them. And David put them off him.
or similar.