You could clean up the entry and exit of the headers a bit with a grinder I'm sure. Never tried it just bought bigger headers. I don't think it will make any difference in sound or horsepower. The big problem is the kinks in the tubes and it wouldn't be worth the trouble to try and make those bigger.
If sound and performance are what you're trying to change, then start with the mid-section of the exhaust first. Either an off-road or a high flow H/X-pipe will give you the most bang for the buck. Then go with the cat back and lastly the headers. Now that you've got the exhaust breathing properly, start on the engine and intake end of things. You’ll be broke before you know it.