Ok, couple of things, using power kits on each other does provide an efficiency boost. Also, if you need rare kits, transforming common kits does actually end up being helpful, but it is long and tedious. Finally, the most efficient way to upgrade kits if you’re using base is for one factory to constantly craft kits while the other three craft commons. Use 19 commons on each common kit, then three max common kits, one rare item, and three common items on each rare kit. Also, slight tidbit that I don’t think anyone else has mentioned, it is actually slightly more cost-effective to upgrade an item all at once, instead of as you get items/kits. The reason for this is that the amount of gold each upgrade costs is rounded up, so if you upgrade individually you end up paying slightly more coins. It’s practically negligible, but good to know.