On our B777 aircraft we were offered and accepted a Specification Change Notice that permitted us to use 15 kts tailwind. As this was during the purchase discussions, it was not a chargeable item.
However, our company operations specifications state 10 kts tailwind limit approved by our regulator. So the aircraft operates with the 10kt limit. That is basically what Boeing is telling you, we have tested 15kts but It's up to YOUR authorities to approve its use.
Mutt