If your car is stock I wouldn't do any mods except for the mod behind the wheel. Too many people think mods will make their car faster and better, but personally I think mods cover up bad driving habits and makes things more complicated for a newbie to autocross.

You have a newbie with a full decked out car, he/she will not know if time could have been made up here and there due to shock compression, shock rebound, sway bar stiffness, boost too high, boost too low, tie bar wasn't taught, not enough camber, too much camber, too much toe, not enough toe. Whereas a newbie with a stock car can just focus on driving and know that it wasn't the car slowing him down but the nut behind the wheel.

Save your money and use it for an Evolution School http://www.autocross.com/evolution I know I thought I was a good driver until I took the school and knocked another 2 seconds off my time on a 30 second course! $225 for 2 seconds, whereas a complete coilover system is around $1000 and that can usually only knock off .5-1 second the most on the same course.