Our limits are 10 knots for both the Classic and 744. IMO, that's plenty.
I don't know why you would want to takeoff or land with a 15 knot tailwind, anyhow. If you look at the takeoff penalties for tailwind, you will realize that it severely cuts into your safety margin. Your chances of blowing tires during or just after takeoff are also significantly increased, especially at high gross weights.
If you have to do it using your emergency authority in unusual circumstances, so be it. However, landing with a heavy tailwind is not something you want to do as a matter of routine.