Considering efficiency as “thrust per watt”, propellers are inherently more efficient at lower speeds than at high speeds. At very low speeds the friction within the motor dominates and makes the efficiency lower, which is why the efficiency is lower under 1600 us.
We use grams per Watt because it gives a very good gauge of efficiency in static conditions, which is what an ROV experiences most of the times. Note that this does not represent the efficiency under moving conditions. For instance, when used on a boat, the thrusters see an inflow velocity due to the speed of the boat. That moves the point of highest efficient and decreases efficiency at maximum speed. Most propellers (like airplane propellers) are used in forward-moving conditions like this, so efficiency is shown in different units than thrust/Watt.