Zeffy would not the actual performance be a gross gradient over the obstacle clearance surface? perhaps it's true that a five or 10 minute limits may make a difference, but usually the limit is most severe on a WAT-limited OEI TO procedure---Engineers like Mutt have even discussed the possibity of of Negative climb performnce for the OEI TO climb to 1500 above the airport elevation---
all the pilot can do is assure that the WAT limits for approach climb are legally met