I was wondering just how much better protectively was later 14th to 15th C full plate harness in comparison to earlier splinted styles/CoP/Corrazinas? Would an earlier style hold up in a battle if one were faced with more up-to-date armoured opponents? There must have been a legitimate reason for switching to more 'metal' armours other than simple weight/fatigue/speed etc trade offs, is it prevalence heavier, higher impact weaponry or something else?

There is no real difference in the level of protection. Armour was made as thick and heavy as it needed to be to stop the most common threats. Plate armour stopped the same weapons as lamellar, mail, and CoPs. Even leather and cloth armour provided the same protection as plate. Plate was more effective because it was lighter than those other armours and it could be articulated to cover pretty much all of the body. Only mail provided more coverage, but it weighed more than plate, was more susceptible to blunt trauma, and cost more to produce. Plate only started to provide more protection than those other armours when firearms became a threat.