So, out of boredom and sitting on a full inventory I decided to max out the machine upgrades and wanted to share my conclusions:
It's not worth it - I already suspected that. They do get a bit better, but never good enough to make up for the material invested. Especially since you need gold and Kimberlite and they don't give you that, so you'll have to actively mine anyway.
But I'm not disappointed with the drops, because upgrading was so prohibitively expensive that I was guaranteed to run out of any use for the loot once I was finished!