DS2500 must be road legal, its got to be the most common Ferodo pad out there....
With regards to swapping rears as well, then its up to you and how hard you brake on track. I switched to 8pot 356mm ksports on the front with standard WRX 2 pot rears and it was a massive improvement, but the car was obviously much more nose heavy on track - it made trail braking into corners fun as the rear end went light
Now running brembo rears with decent discs/pads and it has balanced the car much better. But brembo rears on a WRX are a pain, as the handbrake/hub assembly is a different size. I've got the godspeed handbrake shoes in mine as an adaption, and my handbrake is useless when they get hot after coming off track.