Testing transexuals pre and post change will probably not be helpful. Hormones exert their effects early in life. Look at women when they go through menopause (the turning off of female hormones). They stay "women". Also, for people desperate enough to undergo such a change, they may have already, in utero, been under the influence of mismatched hormones. They may not have been true to their physical sex from birth.

A better study would be to neuter one same sex twin at birth (or better yet, in utero) and then compare!