fruitbatflyer,
Fine with me until your last sentence - yes, it is guaranteed (within the certification assumptions and gross-to-net reduction) to meet 1.6% net at all permissable WAT limits - it just may not be enough gradient capability in the presence of obstacles. You are not allowed to load over WAT in any circumstance without specific regulatory approval.
Usually, the Performance section of the AFM will give you a Reference Gradient chart that will tell you the OEI gradient capability for the prevailing conditions.
Stay Alive,