Here is an interesting article describing the effects of tailwinds:
The summary is:
While each one knot of headwind component improves landing/takeoff performance by about one percent, each knot
of tailwind component degrades landing/takeoff distance by about three to five percent. Tailwinds are roughly three to five times as effective at altering landing/takeoff performance than headwinds…and the alteration is not in your favor.
Tow pilots should think twice about taking off downwind even with just a slight breeze. Likewise landing downwind to speed up launches needs to be done very carefully. Glider pilots should be understanding.
Posted by Andy for Tim