The planet killers are terrible for collecting iron but they are amazing at stone. But you need to look at the bonuses for it. high temp sun in a galaxy with high magnetic field strength. otherwise it's just not worth it. That's what I used them for though. Stone can be broken up into elements required to manufacture the 2 rare crystal\metals (Can't remember their names) Selling those again, especially the green gives a lot of money.
Yea, gas giants produce very little minerals. Terraforming large, 200m extractors at least, 500m preferred, outstrip the gas giant even if you only collect every now and then. I usually got them to 51|51 and that worked. Still needed a ton of them