Good god - people on I-club giving sensible advice *looks over his shoulder to check for the second coming*
I would agree that the best way to spend your $500 is driving lessons.
The 20mm sway bar will up the performance on your car to the level where with your experience you are going to get yourself in a great position to wreck the car.
The reason cars ship here with 13mm bars is that is they are much more forgiving to lift off oversteer. Basically if you some in to a corner a little to hot and mash the brake to bleed off speed - the car will try to swap ends - very close to the limit even too rapid lift of the throttle can be enough to start the transition from understeer to oversteer.
At this point if you don`t know it`s coming / don`t know what to do you are probably going to end up as one more accident report on the subaru insurance record.
If you don`t know what all this means - you should take some lessons before driving your car to the limit.
That would suck as you seem like a fairly decent person so far ;-)
Oh that and I pay too much insurance as it is ;-)
Good luck with the driving lessons - and DO do some autoX - it`s one of the best ways to safely develop your skills as a driver on the cheap. Plus it`s a cracking way to meet more experienced drivers who may ride with you and help and give you free lessons ;-)
Enjoy your car - but do it safely ;-)