I have a 5.7 pistol that I would prefer to shoot suppressed. Will a 45 caliber suppressor be any help at all or just a waste of time? I have the correct threaded adapter so no extra expense. Other than being large and heavy, are there any reasons not to do this?

Define “waste of time”. What are you looking for? Will it provide some suppression over no suppressor at all? Yes. How much? Unless someone has actually metered it, no one really knows. Common sense says that firing a .22 diameter bullet through a .45 caliber hole won’t give that much suppression, especially for a supersonic bullet.