I've been looking for some data on penetration, and I only found the relation of kinetic forces in relation to penetration.
According to this website \/, spears have more than twice the kinetic energy as swords, so they should have at least twice armor penetration of a sword. This points to 25%-30% range. But personally, for balance reasons, I would go with 25%. To make it different from an axe. Increase cooldowns by 10% is a great idea too.
https://myarmoury.com/talk/viewtopic.php?t=25576Soft armours require about 100J to defeat with a blade
Javelins might reach 250-300J, and might defeat arrow-proof armour. From literature, we know javelins are good against armour (Roman sources praising the pilum for penetration, though to Conquistadors noting the effectiveness of javelins).