We got to meet up markeyboy mate !
On an S3 changing to a 3" DP is the better option...
Then chose a decat or sports cat... subjective performance gains though between the two so a sports cat in a 3" DP will give the best gains/legality balance...
<tuffty/>
May I ask why you opted to go for a custom system as aposed to off the shelf like Miltek? Curious.
What diameter are the standard systems on the S3? What are the main restrictions?