I agree, it does shrink the capes slightly if you use boiling water. I find the best method is quite warm water in a flat bottomed dish/bowl, drop it in and leave it. Takes longer to flatten out and wont get really bad creases out like the boiling water, but it doesn't seem to shrink the capes either. With that method as well, the cape goes very flexible in the warm water so settles very flat on the bottom, then as the water cools to room temperature, it stiffens again and is generally nice and flat.
I have seen some glue repairs that are quite good and display much better than if it was just left, but have also seen some that basically ruin the cape. I think invisible tape on the inside is the best because it doesn't permanently damage the cape and reinforces the rip so it doesn't continue to tear.