I honestly wouldnt worry about eating too much soup. If a full tin fills you then eat a full tin. Its no good being hungry and wanting to snack later on. Its better to eat more soup and not want to eat anything else, than not having as much and being hungry. It will only make you feel miserable.
You say you like Heinz soups, have you tried the weight watchers range? I really like the tomato one and I can eat a whole tin of that.
When you have your fill things will change and you wont be able to manage as much.
If you are still concerned, give your band team a quick ring just to clarify what is on the sheet.
xx