Has anybody ever done a test to determine if and how much of a difference in heat each of these mufflers produce while running? Results?

    It's highly variable between specific mufflers, so, it's more-or-less impossible to answer in general. In any case, you choose the muffler or settings that make the airplane fly the best/engine run the best, regardless of the temperature. If it melts or seizes, then it won't fly the airplane very well; Otherwise, its OK.

   You can make tongue mufflers either more or less restrictive or retain more or less heat by blocking some of the holes. This was a very common technique back before mufflers became obsolete.

     Brett