There really isn't much you can do.
Kurt
Id have to agree with the cats as well. Though I'm fairly sure you'd have to be running the factory smog equipment as well to pump air to them to help the catalyst process. Not sure if you're car is still smogged or not, havent read your thread in a while. I know even on my stock engine car when I threw on an H pipe with two "high flow" cats to pass emissions, it quieted it down to boring levels.
I have the magnapack mufflers and I had an O/R H pipe and it was beyond extremely loud. You couldnt talk w/ someone in the car. I put on an Catted X pipe and it quieted it down A LOT and it made it sound soooo much better anyway. Cats will help a lot w/ the sound level.
LOL. You guys crack me up. The corsas are no drone, and fairly quiet idle, LOUD startup and WOT is like .. 4Harleys. Everyone knows I am getting on it. My wife heard me a MILE away getting on the expressway guys.
Like you I'm running Corsa's with the expectation that I would have a quieter system. My exhaust guy has tried resonators but there was no noticeable change. He and I am now of the opinion that an engine with a blower, cam 2 1/2 pipes with 3" tips is just going to be loud off idle. I'm thankful that the Corsa's eliminated to drone that I had with the Magnaflows.