What is a matter with you people suggesting bigger rear sway bars?!? He's losing control of the car with a 13mm bar!!! Add an 18 or 20mm bar and he's gonna be dead and soon.
Learn to drive the car at the limits in a safe environment like an autox or autox novice school or spend money on clear corners and stereo equipment, etc. Tires are always a good idea regardless.
jImp, I'm really not trying to be mean here, I am really concerned for your safety. A larger rear bar will not only make driving "spiritedly" more dangerous for you but also when you're in a emergency manouver and especially in rain and snow.