So, you're preparing a partnership return... Do you have a W-8BEN and W-8BEN-E from these nonresident members?

Where in Article II do you see these withholding rates?  Article II refers to the types of taxes that are covered by the treaty but not the withholding rates.  You must refer to the article that governs the specific type of income to determine the rate applicable.

Isn't it a bit late to deal with withholding when that should have been managed during the tax year?