There were downsides (A project failed because I didn’t have time to fill in the shortcomings of a teammate, which I’d been quietly doing), but on the balance it was very positive.
My tentative plan is to get promoted first, then step down to 80% - that way the pay cut is even smaller relative to now and with a bigger tax saving.
Aside from the other concerns mentioned, I'm currently working on "what would I do with an extra day per week" - which is a deeper lifestyle question and not at all related to work, but still very important for getting the most out of this change.
Another consideration is whether you would lose benefits under a certain threshold. I almost guarantee that HR will not know off the top of their heads, so before spinning your wheels, on fuzzy lifestyle questions you should start the process of getting official documentation on how things like healthcare and vacation days will be handled, it might make the decision a no-brainer.