I've owned my GC8 since 2005, I've had it serviced every 6 months, and I've never had to have the rear brake pads changed. That's how much use they get for street driving.

If you upgrade to 2 pot rears, you will need the dog bone adaptors, and I'm on the belief you have to upgrade to 02-07 WRX rear rotors.

Spend the money on nice set of front rotors and pads