For a non-US resident, how much will IBKR withhold for listed-preferreds payouts (say C PRK)? For common it is 30%.
If the determination of the source of the income or the amount subject to tax depends on facts that are not known at the time of payment, you must withhold an amount sufficient to ensure that at least 30% of the amount subsequently determined to be subject to withholding is withheld.
Given the 30% withholding tax, I find only securities paying “portfolio interests” attractive.