A negative gradient will definitely help a plane accelerate to takeoff speed, but I'll throw this one back at ya - hopefully the mission was a success and you're probably gonna want to land the plane back.... on a downward sloped runway. That's another problem that'll need solving
Not saying its impossible slamming on the brakes throwing out all sorts of spoilers/reversers/drag chutes and what have you to bring the plane to a stop on a downward sloped runway.... but surely it'd be much more cost effective and efficient launching a plane to meet up with a tanker to top up on fuel, and once the mission is complete not having to worry about whether the aircraft necessarily has the performance to return to base on a negative gradient runway?