Lee, you could achieve 300 with just a remap on an STi which Prosport is 675.00. But to get the best noise and performance decat and remap is a must and depends on the system you have but budget around 1500-1800 pound.
The noise is awesome, heard mine on the rolling road while they were mapping it and it was amazing with nice pops and bangs. Listen to it at the weekend at Tatton, bring on the sound offs....[y] Mine is running around 290bhp now and has gone from 250ishbhp.
Oh and i still have got my car will be later today