The coat acts as an insulator. I have a long coat and a stock coat. Guess who pants more? Short coat Onyx! I think it depends on the temperament of the dog. Onyx always has to be busy, and is more anxious, excitable compared to Kacie.
I see most long coats as more mellow, not as hyped. But this is of course only my opinion! I have the same weather as you, but across the lake.

 

We live in Florida and the dogs learn to handle the heat. It's important to watch them for signs of stress and provide them ways to cool down but it is certainly nothing that can't be handled.

Some of it has to do with temperment and conditioning. More excitable dogs can overheat more than dogs that just chill. I also think darker colored dogs get hotter than lighter dogs. Also, understand that dogs that live inside with air conditioning are more likely to have trouble with the heat than a dog that lives in a kennel because they are not used to feeling the heat. Not saying you should put your dog outside, but it is something to be aware of.

I don't know if there is any difference between long and short coats.

 

I live in GA with heat and high humidity. I fostered 3 pups with stock coats last summer and they couldn't stand the outdoor August heat. Whenever the backdoor was open, they were the first ones in.

Now I own a long coat and she could care less about the heat. She'll run all day if I let her. I insist on her coming inside if I am not out with her so she doesn't make herself sick.

 

My opinion is that "in general" long coated dogs aren't any different than a "regular" coated dog as far as weather goes. I have had 3 long coats (all WITH an undercoat) and 4 "regular" coats and have seen no difference. (I have also lived in 3 different climates. Western Washingont with the cooler temps and a LOT of rain, Eastern Washington desert, HOT and almost NO rain, and I also lived in Columbia SC for 3 years with high temps and high humidity.

2 of the long coats and 1 of the "regular" coats were/are also solid black. And my black dogs haven't been any more likely to get "hot" than the black and tan or sable ones.

 

i had a coatie many years ago. at the time i lived in Philadelphia.
it got rather hot in the summer. we had our 90 degree days. during the real hot days i didn't to much with him. we took our long walks at night. i think you have to be carefull of what you're doing with any dog in the hot weather. when it's very cold you have to be carefull also. when it comes to shedding you can probably make yourself a new wardrobe with the hair. they shed all of the time. April through July were the heavy months. we just vaccumed the house and brushed him every other day.

 

Generally speaking there is probably little difference between a "coat" and a "regular" length coat in so far as insulating capability is concerned. Their coat insulates against heat & cold. Of course a lot has to do with the dog but for the most part GSDs tolerate the heat fairly well as long as they are not over stressed.