I disagree, upgrading 2 out of 4 is not a "toggle" unless I only want to use 2 out of my 4 slots at any given time. This "upgraded" slot should behave just like the "upgraded" craft slots, where I can turn it off at will. otherwise I've paid 1000+ gems for something that is a net negative in many scenarios.
What I would propose to fix/improve this new feature is the following:
1. Include a toggle to turn it off and revert back to vanilla cauldron slots as desired, so old crafting recipes can still be used.
2. Instead of duplicating the highest gold value item (which frequently is NOT what I want an increased % chance of receiving), instead duplicate whatever is in the first slot of the cauldron. A, it simplifies coding because you don't have to figure out which item is highest value, and B, it gives control back to the user, allowing us to increase the % chance of getting what we want, in exchange for a slower fusion time because only 4 unique items instead of 5.
Right now, the extra slot is only a true benefit if you do not do mixed-item fusions, which is not the way end-game people use the cauldron.