Supersprint are currently developing a cat back system for the 23is & I think for the newer 2.0 turbo cars as well.
Go on to their website & click their icon for facebook & you'll see some pictures of them geting started with it.
I have been in constant e mail contact with Alessandro at Supersprint over the last six months waiting on him confirming they were going to do a system for the 23is model & it's taken ages for them to get going with it.
Anyway, he has said only yesterday that there will be no real power or torque gains with a cat back system but that the sound would be deeper than standard exhaust & as yet no idea of prices.
They are also going to be doing a manifold & sports cat but only for the left hand drive models as apparently the steering column on rhd is in the way.