The problem with Shimano pad is that the hardness of the rubber, is a little too hard. They are generally durable, but the stopping power is not that great.
Other pad maker tend to offer hard and soft pads. Softer pads have better stopping power, but are not as durable. So if you want a softer pad, just be ready to carry spares (not a bad idea any ways).
You might want to add the drum break and use it as a drag brake. On long descents, I used to worry about glazing my brake pads or over overheating my rims. You can also crack a rim from the clamping force from a V-brake. My current bike was built with bosses for cantilever (or V) brakes, but there is also a boss in the rear for a disc brake. I use the disc brake as a drag brake on long descents.
I will bet that long descents on a loaded tandem are scary. That drum brake is set up to safely radiate the heat from sustained breaking. Go with softer after market pad if you want, but I would also add the drum brake.