I agree, the mythbusters may or may not be valid for a dump trailer. I expect it wouldn't make a lot of difference either way, and like others have said, the trailer is designed to have the gates closed.
I believe mythbusters did two episodes, the first was just the tailgate up or down. The second one was the different mesh, vaned etc aftermarket tailgates, plus various bed covers. As I recall, nothing beat the stock tailgate in the up position. I was thinking even bed covers made the mileage worse, but not by very much.