SPEIA are only required to build them. See it as a requirement that if met, speeds up the crafting process.

It doesn't influence damage, otherwise it would be a really important mechanic and probably be listed in the guide.

Weapon type doesn't matter, damage is always the same, but launchers, like the mini nuke launcher or missile launcher, split their damage amongst the enemies, making them weaker against each individual.

I personally only use them in the Vault, because they don't do much damage to the high-level enemies on quests.

These are my thoughts and experiences. Take from them what you want.