I think this exercise is potentially dangerous because of its ballistic nature. If not done properly a lifter can injure his back etc. I would shy away from this one. I believe all 3 heads are worked in the overhead press. First give your true measurements when ordering one.
Some people exagarate their size when ordering.
If it's too big, it will not work as well as it should, I would suggest using it 3-4 weeks prior a meet for reps 3 and below. Put some talcum podwer on your body before putting the shirt on to make it easier to get on. I would suggest trying it on when you get it before trying to bench with it to get a feel for it. Use a very lite weight at first. Really tight shirts will not allow the user to lower the bar to the chest.