I think desperation is pretty apt because a dual channel tailwind landing in the NG isn't a pretty affair. I wouldn't want to test 15kts in anger, certified or not.
You are always restricted by the PI index. Boeing recommends a further 140m for F40 when dual channel. We have max landing weight charts for some destinations in relation to dual channel approaches. Any degree of tailwind restricts the hell out of the aircraft. If it were me I'd be looking elsewhere...
Equally we have some destinations that allow for 15kts tailwind landings. Otherwise company policy is 10kts. For takeoff max 10kts. Yet always respect the PI for landing or performance manual at the planning stage. Equally the RTOW for departure. Fairly simple set of rules.