If heat is the issue, you might consider looking into an Anatolian Shepherd. They're true LGDs, but they're Turkish in origin, so they're short-coated with built-in heat tolerance. In fact, they use them in Africa to guard goats from Cheetahs (CHEETAHS!!!). We got a Nubian buck from a couple that raised Anatolians and lemme tell ya...up close and personal, they're very impressive dogs.
As for GSDs....we have one and I'm pretty sure she'd kill chickens if she had a chance. I was discussing this very thing in another thread with another member who has a GSD that actually acts as a guard, and we agreed that it's all about the lines from which the dog was bred. As the other member put it...GSDs are bred pretty indiscriminately these days. If you're looking to get a GSD to guard, be really selective and then TRAIN, TRAIN, TRAIN.
Personally, though...as the very proud owner of a true LGD...I'd find an Anatolian pup and let it grow up to be exactly what it's bred to be, instead of taking a chance on having to 'modify' another breed of dog.