The purpose of the lower cast wall gasket on the Austroflamm Integra is it creates an air seal. That seal forces air to go over the top of the upper cast wall which gives it maximum exposure to the heat exchange tubes.
Without the lower cast wall gasket air can go around the side of the lower cast wall and out the exhaust ports with very minimal exposure to the heat exchange tubes. This decrease in efficiency means less heat coming out of the stove.
You may not be able to tell the difference in heat by standing in front of the stove. Where you would probably see it is in the amount of pellets you use through the entire year.