I'm a large B, KT tape managed to make it look like I have a moderate case of gynecomastia (which was sort of okay because I am obese) but it certainly didn't make me look flat in the slightest. It took a hell of a lot of tape too, which is EXPENSIVE to be using so much at once. And in the event you place one piece of tape wrong, you'll most likely have to take all of it off, because with misplacement it can seriously start to hurt.
Taking it off is a chore as well, especially since with larger chests you have to use more tape. You have to use baby oil to loosen it for 20 minutes before taking it off, or else you risk serious blistering and tearing of the skin. I learned that the hard way.
Basically, for anyone above a small B, open-chest binding really isn't an available option. It stinks, but I just resigned to the fact that I'll have to sweat more if it gets hot.
Another word of advice, tank tops over a binder can smooth out lines from the tri-top. As a whole I generally avoid t-shirts unless it's a short trip though, because t-shirts tend to stick to the chest and make it very visible that I'm wearing a binder.