I finally figured out the secret to getting the amount of water the box tells you to actually be absorbed by the oatmeal in a microwave. I cook for 10 minutes on 40% power or 13-14 on 30% power. This will depend on the power of your microwave, of course, mine isn't that powerful. So, if you have the time, this method works well. I too like the massive volume of food for less calories.
As for the cornstarch, I really don't think a teaspoon a day is a problem, if that works to get the volume and consistency you expect. I doubt you would suffer any ill effects from such a miniscule supplement.
I'm on the "Eat-as-much-as-possible-while-still-losing-weight" diet. It's working.