The efficiency is not only in the storage, it is in the use. If you are allowed to use 6.7kw, but the motor is only 80% efficient, that is *removes shoes and socks* *thinks very hard*
not as much power as if the motor was 99% efficient, regardless of the storage.
The more power you are allowed to use, the more beneficial this becomes.
Also, there is the consideration to where all the additional energy goes. if it creates heat, the more efficient the unit, the lower the need for cooling, and hence improved aero, with smaller cooling openings etc.
Always better to be more efficient.