The buck slows the train on the down grade, but at low speed the 1.25V buck is too much. At higher speed the boost/buck works pretty well to keep the speed almost constant on both grades. It depends on the weight of the cars and the friction of the wheels.
You can estimate what will happen by moving the transformer lever higher 1.25V when the engine is on the up grade, and lower 1.25V when the engine is on the down grade. I haven't tried a long train with 2.5V boost/buck yet, but I suspect 2.5V buck will be too much on the down grade.
