As most have said, the brake package alone is just about worth the $3000. Factor in just the LSD and you've definitely got a bargain with the sports package.
As far as the basic brakes, if I recall they are the same as the basic (non-Brembo) brakes for the 350z, single-piston sliding calipers at 12.6" diameter, I believe. They are notorious for fade and poor pedal feel making brake system modification almost a necessity for a car that's going to see any track time, although they do provide decent stopping distances so they were ok for street use.
The OP mention that the brake upgrade is 12" --> 14", but that completely misses the point. The upgraded fronts use 4 piston fixed calipers and 2 piston calipers in the back- this should result in a huge improvement in pedal feel and -along with the increased size- more fade resistance.
BUT--- I feel that if you think that the $3k upgrade is too much, then for you it might be. If you aren't going to be hammering down twisty roads or taking your car to autocross or road course events, you probably don't need anything that comes in the sport package.
In the end, on this board you are going to see people who want a screaming beast for a car who can't understand not buying the sport package and absolutely hate the touring model. You will also find those who want a sports car with creature comforts and can't imagine why someone wouldn't want a touring model or why they want or need the sports package. Base model with sports package for me..
