For example under some flight conditions some gliders have a lower sink rate when carrying ballast - it allows the wing section to operate on a more efficient part of the curve.
Not quite...
Those gliders -- just like any airplane -- have their optimum/best glide RATIO at a higher speed when at a higher weight. So, if the glider is attempting to conplete a long-distance course in the best time, it is an advantage to be able to fly the course at a higher speed without losing excess altitude. OTOH, it may be harder for that glider to climb in a small-diameter area of lift, because the higher speeds make it harder to stay within the small circle.
On approach, an airliner flys at a slower airspeed at a lighter weight, maintaining roughly the same angle of attack. The power required is therefore less at the lighter weight and slower airspeed.