"The car is very quiet at the moment, does anyone know what the standard Sti exhaust sounds like with the cat/s removed?" - BLOODY AWESOME!
Your car has 2 cats and is the last of the Single-scroll JDM cars (twin-scroll on 03 JDM STi's onwards), one in DP, one in Cat pipe. REMEMBER THAT IS FACTORY SPEC IN JAPAN, DOESN'T MEAN THEY ARE BOTH STILL THERE!!!!!!!!!!!!
I highly recommend Hayward and Scott , but that's biased as I supply them up here!
Glasgow massive seems to like the Meercat stuff, not sure on their prices but the quality looks to be very good.
Russell