Mate.. honestly.
Heres what you SHOULD do if you own a SV6.
1. Replace mufflers (pacemaker, redback, difillipo etc) I have pacemakers on mine in stainless.
2. Headers/Extractors. I bought mine for $395 from the distributor in dandenong south. Then got it coated in castlemaine for $250
3. Cats (if you must) 100CPI cats are the best flowing, probably abit of overkill, id go with ballistic's or even the xplode ones on here. probs around $300-$400
4. Cut out the hotdogs and crimp section in the stock exhaust and replace with stainless steel 2.25" pipe. $50 (if that)
DON'T CHANGE ANYTHING ELSE. If you go out and 'upsize' to a 2.5" or 3" exhaust you will be loosing power and fuel economy UNLESS it's turbo/supercharged.
The exhausts on the SV6's are pretty much perfect cept for what I mentioned above. Don't waste your money on a $2000 catback, it's uneeded.