In the .22 Hornet you're never going to see much difference, even in hot weather, because you can't get enough 1680 in the case to cause any pressure "symptoms." But in the .17 Hornet there's plenty of room still left in the case even with 20-grain bullets in max loads, and yes, you can see signs of high pressure in hot weather.
Seafire doesn't have any idea of what the pressure is in his .204 load, because there's no loading data for 1680 in the .204. He likes to experiment with various powders and often works up to where pressure "appear" safe. It may be a very low-pressure load, but nobody knows.
