Not really vanilla-relevant, then.
Even in the explicit case of modding, doing a blanket OP discount through vanilla isn't a good idea. In any case, there's a method to arbitrarily grab weapons/fighters (if not through design type explicitly, then through prefixes/suffixes, which achieves the same thing) and then modify their OP costs and possibly other things through hullmods. Which is to say, if the mod author wanted their faction's weapons to have certain advantages on their faction's ships such as OP discounts, they certainly could. Most mods don't though, so the authors presumably don't feel any need to.
On top of all the sudden balance implications faction-centric OP discounts would cause, it also means that if the mod author doesn't want their faction mods to be affected by the changes (since most of them don't have any factional bonuses from mounting same-type systems, that'd be most of them), they'd have to start jumping through hoops to avoid the undesired interaction. Maybe by making the ship designs and weapon times technically different (ex: "Faction_Ship" vs "Faction_Weapon") or creating a hullmod whose only purpose is cancelling out the undesired effect from vanilla.
Either way, this would create plenty of issues without solving any.