XDs and RDs are road bike cranks so how could they not be used to put some "Serious Miles" as roadies do more so than the rest who ride fixed. It has absolutely nothing to do with efficiency, it would take a very sensitive testing computer to even tell you the differences between those cranks and how efficient they are so I doubt any rider could tell the difference. Slapping a high priced crank because its NJS approved will not make you a faster rider because of your perceived improved efficiency.