Indeed it does. So, if you have emissions testing, might not want to do it. However, the CAT is one of the most restrictive things in the exhaust...replacing it with the straight through is probably good for about 10-15 hp.
"the CAT is one of the most restrictive things in the exhaust" - actualy it's not that bad and 10-15bhp gains are only possible if you are already pushing much more than stock levels of power - removing the cat on a stock car does not get you 10-15 extra horses. The main performance destroyer on the exhaust is the cast elbow on the turbo. The gases slam into this and get slowed right down which kills spoolup. Also when you remove the CAT the exhaust temp drops as well which also slows the gasflow - spoolup is actualy slower without a CAT. If you are chasing ultimate power then remove the CAT but if you want better drivability and better turbo response - replace the cast elbow.