Lots of good exhausts, but one thing to make sure - get a downpipe also. The cat is bolted to the turbo on the 1.8 motors, then there is a downpipe that is not always included with the "catback" exhausts for this car.
The full system is needed to get the best benefit. So look for the downpie too.
Borla exhausts are one of the best.
VB