Right now it these thing provide a boost of 300 defense and go up to 7000, from the previous cap of 2100 from Gravitic Amplifier that's 17 uses per planet. Assuming you get this mission once every 2 weeks and are on rotation with the rest of you legion, 1 every 6 weeks ~2 years to max 1 planet.

Can these be increased to 1000-2000 added defense?

This is a few misions Items what need a little tweeking like this one. +1 to the Idea.

Just because most ppl(myself included) use Magnetosphere Charger for cloak doesn't mean you can't use it for def. I'm too lazy to calculate out what is the actual max with the charger, but 3800 should be doable. That means, at 300/use, you are going to use 11 of them.
that said, it's still too long and tweaking would be wonderful. Just wanna set fact straight.