Is there a minimum tax guarantee that limits how much a section 217 election can cost me?

Not exactly, in the sense of a built-in floor that automatically limits how much a section 217 election can cost you. The real protection is that the election is voluntary rather than mandatory: nothing requires you to elect, so you're only meant to make the choice after comparing the graduated-rate result against your existing flat withholding, and if the comparison shows electing would leave you worse off, you simply don't elect and the flat withholding stands as your final tax for that income.

Once you do choose to elect for a given year, though, you're generally bound by the result of that calculation for that year, there isn't a separate safety net inside the election itself that caps your cost below whatever the graduated calculation, based on your actual worldwide income, produces. The protection here is really about making an informed choice upfront, not about a guarantee built into the mechanism after the fact.

Because the outcome depends entirely on your specific income picture, and because this is a year-by-year decision rather than a one-time commitment, running the actual numbers before deciding whether to elect for each relevant year is the only reliable way to know whether it will help or hurt.