Yip you will need it for the s3 cooler, if its the alloy pipe ! Infact if your car is an 06 plate or below ( depending if its a late 06 or early 06 plate ) you may need a differant pipe altogether to fit the s3 cooler.
The s3 cooler is a great upgrade for the K03 IMO
Mine was a late 06 plate but I bought the pipe just incase I needed it, but ended up not using it.
So I've got a brand new unused one here - if I remember correctly they were about £44 from the dealer, you you fancy mine £20 delivered