So we are on the same page. Maxing a power kit (PK) before using it always worth it. If by upgrade you mean upgrading a maxed common PK to rare and maxed rare PK to epic. Then the answer gets muddy.
Assuming you have unlimited time, power kits and gold then it is not beneficial to upgrade power kits. Given one or more of those three resources are limited for you then you have to find what works best given your resources.
I have used power kits in so many ways. Here is currently how I use them. I will upgrade common or ideally rare to a maxed epic. Then max the rare and common items to finish maxing the item in question.
This seems to be the best use of my time. It is muuuch cheaper than producing power kits form base and it requires very few power kits to sustain upgrading.