Flowmaster catbacks on these cars are terrible, the crossflow 80 series makes no power. The Corsa is farily quiet on LS1's, you could sell it for a good bit of change and buy something else. Adding longtubes will really wake the car up on the top end as well as make it much louder; add in a tune and power will increase dramatically in the low range. Stock exhaust is 3 in diameter, plenty for a stock cube LS1 and then some. The only dual exhausts available are the Kooks (very expensive) and the TSP (dumped bullets). A shop can fab up a custom one, but quality in the bends and piping usually lacks or will cost you a seriouc penny.