Ammo mods don't really alter the overall dmg type a huge amount. Everyone specs energy and piercing. Which is boring.
It also doesn't make much difference when it comes to PvE.

What about if ammo mods changed the dmg type so that the majority was the new type? So about 75% to 25%, rather than about 10% to 90%.

You'd have to think more carefully about your setup and what dmg type to take when hunting.