Shorter cranks make you more aero as you don't have to bend the knees so much so your air resistance level is less at higher speeds so more watts saved... or for the same watts, more speed
I got mine cut down to 155mm but I know others who have gone down to 145mm. I enjoy a little leverage and it works for me. After 100 miles my knees don't hurt.
It also helps having your cleats well back or in the middle of your shoe
