in december i lent my sis-inlaw the car and having two small kids with had to remove any panels we could to line it with sound proofing, heat wrap all the exhaust and backbox/cat. knocked it down a few dB's but the result on the outside has gotten a bit louder due to the wrap. lol
i wouldnt go for a cat-back system if i had to commute a long distance to work, took a while to get use to it as was making me feel sick and tuning quietened it down a bit