Crush your topspin serve.
Use a maximum windup and hit it as hard as you reasonably can, and let the topspin take it down and into the court.
If indeed you grow some more, all you will have to do is move the toss slightly more forward and then you can crush your flatter topspin serve.
Watch all the players at the US Open.
No one is hitting a perfectly flat serve.
Everyone uses plenty of topspin (and a little slice) on their first serve.
Notice how much they arch their back so they can get hit over the top of the ball, spinning it down and in.
To get a bigger windup, and therefore more racquet head speed, make sure you are dropping your raquet vertically down, and getting it quite low at its lowest point. Do this by protruding your forward hip out (forcing your hitting shoulder down.) As you spring up from this position, your whole upper body will be thrown forward (while your chest is still pointed up to the sky), so it is not just your arm "muscling" the ball for pace and spin.
http://www.fuzzyyellowballs.com/vid...-technique/leading-with-your-hip-when-serving