I've watched them. They used to have weight pull competition here every year (in the winter - weighted snow sleds) but stopped. (they were mostly husky/malamute/alaskan mixes) Are you thinking about a GSD for weight pull? Wouldn't that be a bad idea with a herding dog with a history of rear joint problems?
Kaze, my GSD, can pull as much, if not MORE weight, than both of my huskies combined. Ha! Nara, not so much. She's a dainty gal. But my KING Kaze blows my mind away with how strong and fast he is. I've heard of sled dog teams made up of all GSDs. It's rare, of course, but they're out there. You just have to start small and build them up. I've had Kaze pulling around 360 lbs and he can go faster than the huskies pulling about 60-150 lbs.