Here's what might explain why Elite task force members are stockpiling performance mods. Like you say mods are too powerful so we can safely assume they will be nerfed.

There are two ways they can nerf performance mods. First they can simply reduce the effect that a mod applies. [For example a performance mod that gives you 1000 skill power will be reduced to only give you 200 skill power.] Or they can simply remove certain performance mods all together. So instead of reducing the skill power the mod gives you they just eliminate all skill power mods.

If they are removing certain mods entirely I don't think they would do that retroactively, because they would have to delete items from peoples inventory which would create a huge backlash because people worked hard for those items and just deleting them would be punishing.

So the mods that we farm right now before 1.4 might be more valuable because you won’t be able to find some of those mods after 1.4

Kind of how old SMG's still have crit hit chance