Neglecting friction, from a physics point of view the ONLY thing that matters is the starting and ending hight above sea level. If we assume a perfectly flat road starting and ending at sea level the total work done (neglicting friction) is zero. The story is exactly the same for a road with hills or curves, so long as you start and end a sea level.

In practice we must overcome rolling resistance, and mostly air resistance. The speed limit requires the need to use engine braking or friction brakes on the way back down a hill. The only way to get the energy you put into the system by going up the hill would be to coast down the hill at well over the limit and even this is not as efficient because the higher speed coming down would generate more air resistance, even without regard to speed limits because the drag force is proportional to the square of the speed.

A level road allows the speed to be near the limit without the need to go over to recapture the energy.