Captain Obvious!
I didn't see a single post that anyone said a Lab or Chessie could out run or handle the heat better then a Pointer or Spaniel. That goes without saying. Not everyone runs dogs in hot weather. Not everyone hunts birds hard day in and day out. Not everyone wants a Pointer or Spaniel for a dog. If a person wants a Retriever most likely they already know what they are getting into and looking for. Some people just like a certain breed. If it fits their bill for hunting so be it, it's a perfect match at that time.
However I will tell you that a well conditioned, trained, and fed Chessie will out run any person all day in the field. Heat can and will effect any dog. Some more so than others. They (chessis's) are not covering the ground a Pointer is expected to. They should be close working to flush for you. I will say that a Chessie is a bull in a china shop when it comes to hunting Ruffed Grouse and Woodcock. For sure a smaller more agile breed is better served.
One thing generally speaking to remember: Small dog, small bills and Large dog, large bills.
