The Track Pack will absolutely give you not only the best acceleration, but also suspension and chassis upgrades that improve the handling. You will also get beefier brakes, all for around $1,800. Considering you wouldn't even be able to get the brake upgrade for that money, it's a no-brainer.
The only thing to consider is the fact that you get summer only performance tires with the package, so you wouldn't want to drive the car in temps below 40 degrees or so, as the tires will get harder and perform poorly. Forget using them in the snow.
The reason you haven't been able to build it online however, is because it isn't available yet.
D'oh!!! It will be available later this summer, but for the money, and the all around performance improvement, it's worth the wait. Your acceleration, handling, and braking will all improve drastically, not to mention the overall balance of the car. If you can afford to wait, I
highly recommend it. If I was only gettin one option on the car that would be it.
I work at a Ford dealer, and would be glad to post here when the option becomes available.